• Elita One 27.2mm seatpost: I've bought like 5 of these for various builds. Hard to recommend much else. Light, well-made, can handle a load, no branding. https://www.aliexpress.us/item/3256805646314200.html
• Faceted anodized top cap: Well made, nice bit of bling on a bike with an appropriate color scheme. I have the rainbow one on my Tarmac and it's been complemented more than once. https://www.aliexpress.us/item/3256805632252290.html
• Reflective tape: I put some on the rims of my commuter bike for side-on visibility and it works great. Cheap and adheres well and is VERY reflective. https://www.aliexpress.us/item/3256804860946991.html
• Cyclami TPU tube: I use these for my commuter bike where I don't want to bother with tubeless. In the 27.5 size they hold air very well. The rear tube suffered a small tear (not puncture) as I was installing a new set of tires, but I patched it with one of the included patches and it hasn't bothered me since. https://www.aliexpress.us/item/3256805483094623.html
• ZTTO P3 hubs: I've built 3 wheelsets with these hubs, they're my favorite cheap hubs. They roll well and have good weather sealing. The measurements in the listing are accurate for wheelbuilding. Not super light but not egregiously heavy either. Of note is that 12mm front thru axles will need an axle reducer (https://www.aliexpress.us/item/2251832711018601.html) and the front endcaps will probably need to be filed down to accommodate the shallower dropout cutouts. https://www.aliexpress.us/item/3256802437891171.html
• Kalloy UNO stems: I have versions of these on all my bikes. Light, cheap, strong.
• XCADEY chainrings: I don't have much time on these rings but chainrings tend not to change very much over time so I feel comfortable recommending these. They shift very well (50/34 at least), are light enough, and imo look pretty good. https://www.aliexpress.us/item/3256803817933289.html
• RYET 3D printed saddle: tried one on my road bike and liked it enough I got another one for my gravel bike. The shape is basically a copy of the PRO stealth which works well for me. The claimed weight is accurate and the construction quality is excellent. https://www.aliexpress.us/item/3256805023659024.html
• PASS QUEST jockey wheels: a nice little upgrade from most stock pulley wheels. Durable, narrow-wide, with good bearings - sealed, not the crappy ceramic bearings completely exposed to the elements that most Aliexpress jockey wheels have. https://www.aliexpress.us/item/3256801862899014.html
• Spcycle MTB handlebars: generic carbon MTB handlebars, but very well-made with a clean interior. Plenty strong, I've taken these down some real jarring singletrack with no issue. https://www.aliexpress.us/item/2251832093175674.html
• ZTTO nipple wrench: the little mickey mouse ears are so comfortable to use. By far my favorite nipple wrench, better than the Park Tool one I was using. https://www.aliexpress.us/item/2251832816974319.html
• DECKAS 1x chainring: I've used 4 of these on various 1x setups and they work great. So cheap too. I wish the branding was slightly more subtle but it's not terrible as is. https://www.aliexpress.us/item/3256801606393029.html
• Generic flared gravel handlebars: Shockingly durable and comfortable. I like that the come in narrower widths. The internal routing is clean but not very wide - fitting shifter housing + brake hose through each side was a royal pain. Unfortunately the underside isn't really shaped for external cable runs. https://www.aliexpress.us/item/3256802785082557.html
• Flyxii FE-02 gravel bike frame: a real tank of a gravel frame. I love mine. https://www.aliexpress.us/item/3256802462426945.html
• SERAPH road handlebars: my favorite road handlebars ever. Such a nice shape, well-made, light and stiff, nice channel on the underside for clean and easy cable routing. I hope they never go out of production. https://www.aliexpress.us/item/2255800581774460.html

• Generic front light: I think this is the same one sold by "Offbondage" (lol) and a few other stores, but this one doesn't have branding which is why I went for it specifically. Nice and bright with a flashing mode, has a good bar mount, USB C charging. https://www.aliexpress.us/item/3256804897699962.html
• OG EVKIN HB010: good shape, reasonably light, external cable routing for easy setup. I have this bar on my commuter and it works well. Only reason I don't rate it higher is because the first one I had snapped when my bike fell over in its stand - can't really fault it for that but my other carbon bars have survived their bikes tipping over. https://www.aliexpress.us/item/3256805458531947.html
• Generic bike baskets: I use 2 of these on the front rack of my commuter for light but bulky cargo loads. Not super well-made but they do the job and fold down for storage off the bike. https://www.aliexpress.us/item/3256805376389581.html
• Sunshine cassettes: I've had 3 of these; 2 11-speed (11-36 and 11-42) and one 12-speed (11-46). They shift fine, not quite as good as Shimano but that's to be expected. Very heavy though. https://www.aliexpress.us/item/3256803959318100.html
• AVID HS1 brake rotors (definitely copies): 6-bolt only, but they don't warp because they're thick, they brake well, and they're incredibly cheap. https://www.aliexpress.us/item/2251832609689188.html
• Nipple holder: makes the process of lacing a wheel much nicer. For 3 bucks, well worth it. https://www.aliexpress.us/item/3256802890581426.html
• ZRACE Blade MTB crankset: simple fitment and steel axle, same as Shimano hollowtec cranks. Reasonably light, graphics aren't too obtrusive and the included chainring is surprisingly good. I have found that it creaks a bit when it gets dirty but it's so easy to pop off and clean that this isn't a major issue. https://www.aliexpress.us/item/2255800868849020.html
• Spcycle M05 frame: size medium in glossy red color came in at 877 grams - amazing! Despite the weight, it's been a very solid hardtail for my not-very-extreme needs. Only drawback is the rear driveside dropout is too thick for the stock hanger so I had to file down the hanger to make room. https://www.aliexpress.us/item/3256802796251755.html

• TOSEEK 143 saddle: the shape works fine for me. The construction quality is decent but the middle unsupported section tends to sag a bit too much imo. https://www.aliexpress.us/item/3256805099852978.html
• ZTTO MTB cassette: shifts ok, not nearly as good as the Deore cassette I was using on my previous MTB groupset. But this cassette is amazingly light, the claimed weights are accurate, and I think at least some of the shifting issues I have are down to some derailleur hanger issues on my MTB frame. https://www.aliexpress.us/item/2255800094553452.html
• IIIPro centerlock disc rotors: they don't warp which was a major improvement on the Dura-Ace rotors I was using, but after about 500 miles they developed a squeal I couldn't get rid of even with brake cleaner and sanding their surface. Swapped to Campy Ekar rotors and have been very very pleased with them. https://www.aliexpress.us/item/3256804105057808.html
• ZRACE Hardrock crankset: I've used two of these. One has lived on my gravel bike for 1,500 miles, has been beaten up, fully submerged multiple times, been put through singletrack riding and 1200 watt sprints, and is basically brand new. The other one had the axle and driveside crankarm separate and develop play within 100 miles on my commuter where it never saw any abuse. So... ymmv. https://www.aliexpress.us/item/2252799820023948.html

• Ridenow TPU tube: I only use these as backup tubes on my road bike. The one time I had to use it, I was riding close to home with a friend and he punctured with no spare tube, so I threw this in his tire and it flatted within 5 minutes. Fortunately I was able to ride home and bring him a butyl tube! I tried patching it when I got home but it failed with 3 different patches.. So small and so light, but this really is an option of last resort in my saddle bag. https://www.aliexpress.us/item/3256803826508749.html
• Sensah SRX Pro: The shifting was very good, but the clutch on the derailleur was so weak that it dropped my chain several times a ride. I quickly replaced it with a Deore rear derailleur + JTek shiftmate which worked ok. The braking was ultimately the dealbreaker - see below - but I kept the shifters around on my commuter bike, paired with a SRAM Force 1x rear derailleur, and the shifting was great and the braking much better with rim brakes. https://www.aliexpress.us/item/3256802576089351.html

• Juintech F1 brake calipers: I just could never get these to brake all that well. I tried different pads, rotors, preload adjustment, etc etc etc; ultimately I was descending a notoriously steep and winding local road and I nearly lost all braking power, even after stopping and letting the system cool down for almost 10 minutes. I ordered 105 hydraulic shifters and brakes the next day and they're infintely better. Don't bother with these or other cable/hydro solutions, just go hydro. https://www.aliexpress.us/item/2255799928780428.html

Do you know the reach and drop measurement of these bars?

I'm surprised you listed the Juin Tech F1 calipers as bad. I have mine setup on my road bike and they work just fine. I do nothing but climbing and descending as well and I've been lazy to swap over Ultegra R8020 hydraulic shifters I have. Though I'm using higher tension gold brake cabling and semi-metallic finned pads that were released in the last few months. It's made a huge difference in performance.

Did you happen to get the weight on these?

• Internal cable routing tool and an old cable (or just 2x cable routing kits)
• A frame with a headtube large enough that you can a fit Di2 battery in while tilting it into the downtube
• Bubble wrap and tape
• Zip ties, with a length approximately 2.5-3x the diameter of the downtube of whatever bike you're working on

• Drop out the fork and remove headset bearings. Remove all hoses/cables from the downtube to prevent the Di2 wires from getting snagged or tangled. Remove any cable routing guide near the bottom bracket; the holes this leaves will be valuable for cable routing.
• Route the cable routing tool through the top or bottom of the headset, whichever direction is easiest to insert the battery from. If you have 2 cable routing tools, use one for the front and one for the rear; otherwise use the tool to route an old brake or shift cable from the headset through to the rear derailleur. If possible, route these under the bottom bracket.
• Wrap the Di2 battery in 3-4 layers of bubble wrap and tape to keep it in place. I used silicon self-fusing tape but any durable tape should work. Wrap several zipties around the battery in different directions. I used 8 zipties, spaced a quarter turn apart (i.e. 2 pointing in each direction).
• Attach the Di2 wires to the cable routing wires (I used a little strip of packing tape which worked fine) and the battery. Pull the wires down into the downtube - make sure to do this evenly, i.e. pull the front and rear wires the same amount. It should be pretty easy to do this up until the battery has to go in the downtube. Lever the battery into the downtube, putting a little spin on it to get the zipties in a sort of spiral pattern.
• Pull the battery down the downtube - this should be pretty easy until the bottle cage rivnuts are encountered. Your rear (and maybe front) Di2 wire should be long enough to be pulled out the bottom bracket cable routing holes and a bit more force can be used to pull on those to get the battery down past the rivnuts. Eventually the battery should make its way down almost all the way to the bottom bracket.
• Finish routing the Di2 wires and tape them to the frame. Shake the frame to make sure the battery is secure and doesn't make any noise. If it does, you can push out the battery back through the headtube with a long, thin dowel or some other tool. Take a light that fits in the headtube, shine it down the downtube, and look through the bottom bracket holes to make sure there's enough space to run a brake hose and dropper cable.
• Route brake hose and dropper cable as normal. I found this easiest to do by using the aforementioned light in the headtube, and carefully pulling the magnetized end of the cable routing tool past the battery. Doing this blind wouldn't be fun. After they get past the battery, I recommend routing the brake hose below the bottom bracket and the dropper cable above it, but this is more frame-dependent than anything.

• Fitting the battery down the seattube below the dropper. This is what I tried first, but the battery has to sit down far enough that it blocks the port for the front derailleur wire, so the battery has to be shifted to the side to make room. Unfortunately, this blocks the needed space for the dropper cable, and also fouls on the rivnuts for the seattube bottle cage and derailleur mount. I got my battery stuck in the seattube and had to route 4 brake cables through the bottom bracket, up through the seattube past the battery, and use the heads of the cables to pry it loose. 0/10, would not recommend.
• Putting the battery in the downtube without zipties: there needs to be a lot of bubble wrap to prevent the battery from moving, but little enough bubble wrap to allow it to be inserted into the headtube and get past the bottle cage rivnuts. I don't think this is possible, and if it is, it's far more practical to use zipties. Just for fun, I tried it, and the battery got snagged on the rivnuts and I had to fashion a tool to yank out the battery. Again, not recommended.

I read one user complain about flex. Any other issues with the frame I should be aware of?

I'm wondering if there is any reason why I can't take the left bb cups and put them on the right side? Same for right side cup. If the threaded inserts are indeed on the wrong side then the threading should work. no?