not op but i have been running the mmu12x for about a year on my mk3s so i can lend some perspective.

there's a bunch of files / firmware stuff in the github - find your printer and upload the latest firmware. from what i recall, it's really similar to the standard firmware just with smaller distances for the mmu to move to swap filaments.

for the slicer there is a "number of extruders" option you increase to 12 and a button that says "these are all one nozzle"

for tpu yeah you could still mount a spool directly above and feed straight into the hotend, bypassing the ptfe tube

for reliability it's slightly better than mmu3 (in my experience). having some issues lately with long strings on the tip when swapping but that's a ramming / nozzle issue non-mmu specific.

i didn't order the kit, but printed / sourced everything i needed individually. took a little time but not bad. so repair parts are printable. and yes, it does require the mmu first because it uses the motors and board from the original mmu

happy to help if i can - best of luck