Palmrests for these units are being sold without the TouchID / power button on the far right end of the touchbar. The...

Thank you for posting this! I've encountered SEVERAL cases in my shop where the Crucial MX500 SSDs in particular do not play nice with the 2012-2020 21.5in and 27in iMacs. The drives will appear some of the time, and work perfectly some of the time, but then give random question mark folders which will resolve on their own. I've also seen massive amounts of lag in our Arch Linux test environment with MX500s installed. (And yes, this is AFTER removing the faulty SSUBX/SSUAX drive that might have been there as part of a Fusion volume.) I have NO idea why that's the case, but it's been consistent enough that we now only install WD and Samsung SSDs in this era of iMac since they're such a pain to open up once you've installed aftermarket seals. I would recommend AGAINST using BX-series Crucial drives in these systems, too, because they sacrifice speed and useful lifespan for cost and, again, it's a pain to get into and out of these and you risk breaking the screen, so investing a bit more for a high quality,...

Definitive answer: You can use the original TouchID button with a new board, or a new TouchID button with the original board, but you'll lose the ability to use biometrics without having Apple do something with AST2, which independent repair shops do not have access to. Some degree of onboard component swapping MAY be possible, but I have yet to experiment with that personally. This and tons of other great information is available at https://logi.wiki !

FYI:

Removing the TouchID button (for palmrest swaps following liquid exposure, presumably) is a pain, and if you damage it at all you’ll apparently need to get a new logic board entirely since the two are “married” as I’ve been led to understand. BE CAREFUL!

The button itself is mounted on a black metal shim-style frame/hinge that provides the button's pop-up action. The hinge's open edge is lateral (farther away from the center) and its connected edge is medial (towards the center of the laptop).

When removing and reattaching the button, you'll see that there are six screws to deal with - four silver and two black. The silver ones attach the button itself to the shim, and the black ones mount the shim to the frame. I found that the lateral black screw needs to be a little loose to give the button room to press in.

Also, spend a good deal of time being sure that your button alignment is proper - it’s really easy to have it shift or rotate slightly from its proper location. Don’t want to have to repeat your work!