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Just chiming in that I'm going through the same thing.  I started with the Lexon mount that looks exactly like that OG Evkin one, it vibrated so much that the plastic insert that holds the Garmin broke in a gravel race.  I ran a 3D printed mount that lasted a while but eventually broke right at the front bolt point, but it lasted a couple hundred miles and didn't vibrate at all.  I revised the design and I'm going to send it out to be printed out of a stronger material than I can print at home.  Looking for an off-the-shelf solution, I've heard the K-edge mount breaks as well.  Silca is real expensive.  Not sure what else is out there.

The bottom is trunnion, so the frame will come with bearings in the frame and the bolts, then the shock will come with threaded holes in the body.  Nothing else to purchase for the bottom mount.

Just an update, ZTTO offered a warranty replacement on the ratchet ring, but said they couldn't make a shipping label unless I buy something else... wasn't going to use it anyway and was going to keep it as a spare, so I just won't bother.  I was surprised I was able to get the old ratchet ring out with how little spline was left, took a bit of heat and a lot of torque.  I purchased the OE DT ring (HXDXXX00N1131S) and got it back together, all is good!  Comparing the ZTTO ring with the DT ring, the splines on the ZTTO ring are smaller, so a tighter fit with the ratchets.  I've got a few ratchet kits laying around, and the Fonvo 54t ratchet actually gets stuck in the ZTTO ring, but slides freely in the new DT ring.

The only hardtail I have experience with is a pressfit BB, so I'm no help there.  But I just wanted to say that I've seen enough negative experiences with DIYCarbonBikes that I'd stay far away.

Just a 200 mile update.  So far I've done some mixed adventure rides, 30 mile gravel race, and a fast road ride with others on true road bikes.  I'm still very pleased with the 707! 

During the gravel race, I hit plenty of serious potholes, stuck with the lead pack for the first 8 miles so it was about 8 people wide and no way to avoid them.  I hit some hard enough to break my bottle cage on the seat tube and the down tube bottle fell out.  Frame, fork, seatpost, and bars are still solid.  Averaged 21mph, this bike is probably 2mph faster than my old Monstercross bike.  This isn't my video, just showing the course, it even went through an actual gravel pit! https://www.facebook.com/joe.miles.9484/videos/1424082484697432

Luckily fit is perfect.  I tend to ride smaller drop bikes (but larger MTB's).  I'm 5'10" with ~31" inseam and long arms, yet opted for a medium with 400/80mm bars.  It fits like a 54cm bike, exactly like I wanted.  I think my only two nitpicks have been the levers being mounted just above the sticky paste on the handlebars, so I had them slide down during the race.  Seems to be fixed with some carbon paste and higher torque.  Also, shifter cable friction may be an issue with the tight bends through the headset.  I first built with Shimano SP41 housing and cheap Amazon cable with no lubrication, sometimes the front derailleur would get hung up going down to the small ring, and the rear would sometimes need 2 shifts one direction, and one the other to make a single gear change.  I replaced both cables with Jagwire cables and used Tri-flow in the housings, the front seems perfect now, and I think the rear just needs dialing in cable tension, only have 1 ride since the change.  If I did it again, I would've just saved up for electronic if I wanted to stick with full internal.

FYI, I just noticed the ICAN AC388 has bolt on dropouts. I don't see details about having QR dropouts available, but that's the kind of frame that would be convertible. May be worth reaching out to them.