HA!  My main riding buddy has the top of the line $7k Look 765RS gravel bike.   He's an automotive mechanic by trade but can't figure out how to properly take care of his chain, despite my offers to do it/show him how.  We will be on a ride and I'll say something to the affect of "Do you wear your ear bud in the right ear because because your chain sounds like a couple of Transformers trying to screw or is it just a coincidence?"

LBS like these need to die. Show me a carbon wheel that isn't made in China. Show me a hub that's not made in China.
Anyway.
Trace velo has a recent video where he thought he got a bad cassette, when it was a MTB type cassette that just needed a spacer to fit on a road hub.

• 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.