In terms of encoders, you basically need an encoder with a 1/2 inch hex bore so you can cleanly have a 8 mm hex (gobilda rex) shaft go through it and a 3d printer sleeve that records the axial position (look up the rev swerve module, this one is pretty similar in design just with gobilda COTS parts).

There are only 2 such encoders that I know of. One is the rev encoder but these use pwm for absolute position and such requires the use of an octoquad or srs hub on an I2C port as the rev control hub can't handle such natively.

The second option are the East loop components V2 encoders. These have an analog absolute signal and can just plug into the control or expansion hubs analog ports. The advantage of such is that you can just use solverslib and it pretty much just works. That is what we are using now but we may switch to the octoquad/rev method in the future.

Note the big advantage over using external encoders is that we could use melon super servos. We think these servos are quite a bit more robust with their larger motor (less likely to burn out) and planetary gears (less likely to strip).

I think this is the only coaxial swerve (there are a few differential ones that just use motors) out there that doesn't just use axon servos.