Assorted thoughts:

Also doubles as a filament runout sensor.

Bearings:

• Only a tiny angular movement -> technically a bad case for ball bearings due to lubricant not getting recirculated, but I think the bearings are being run dry in this project anyway (and are under next to no load)
• I wonder if sleeve bearings would be cheaper but work as well?
• Alternative solution: put a PCB on the left and right sides, then unthreaded SMD soldered standoffs like these as sleeve bearings? I think some are brass under the platings. Exact alignment isn’t necessary as once soldered they won’t move (and you calibrate the device manually afterwards anyway).

Micro:

• ATtiny’s are expensive last I checked :P I prefer STC (8051 clones) but even they’re a bit much these days. A padauk or similar would be an extreme.
• Staying with the Arduino IDE is probably an attractive goal for ease of development, so staying with the ATTiny might be best.
• Comms: I wonder what’s easy to interface with Marlin? Extra UARTs running at very slow speeds (eg a few thousand baud) might work well.

Applications:

• Do you adjust filament feedrate on the fly?
• is it necessary to delay the adjustments by X cm of filament? Or does it change slowly enough that it’s not an issue?
• Detect lumps in filament and pause/alarm prints! I once had lumps of some higher-temp plastic in my roll of recycled PLA, it would jam and cause my prints to fail. Very annoying!

Calibration:

• Using drill bits is brilliant, I love it.
• Do the magnetic properties of the steel cause issues? What happens if I accidentally magnetics my bits? Perhaps I should only insert them from one particular end whilst calibrating to maximise their distance from the hall?

Screws:

• I like using “coarse” 3mm plastic screws. They don’t have a tidy standard like “M3” but you can get them on lcsc and other places. They hold in 3d printed plastic really well, can be removed and inserted dozens of times (surprising but true, even if you cross-thread occasionally) and most of all: don’t need brass inserts!

Apologies if some of this was answered in the video. I’m sorry Mr presenter but you waffle more than I do :P so I skipped a few bits.