Have you looked at using an optical encoder?
With the encoder disk just part of the 3d printed thumb wheel.
Ploopy mouse uses this for their scroll wheel. And Ploopy is open hardware and open source qmk firmware. So you can use their qmk code for optical encoder handling in qmk, their schematic for component selection notes, and their scroll wheel CAD files as a basis for yours.
QMK code: github.com/qmk/qmk_firmware/tree/master/keyboards/ploopyco/mouse
Schematic: github.com/ploopyco/mouse/blob/master/hardware/electronics/schematic.pdf
CAD files: github.com/ploopyco/mouse/tree/master/hardware/mechanicals/
