Great update, thanks for sharing.
From my experience (experience is what you get, when you don’t get what you want) with Time-of-Flight sensors, I did try a pure math approach to moving without capstan, and it worked fairly well, except one my my goals of the project was to scan film without sprockets (develocorder), then loosing a position reference to work with.
Using 100:1 geared steppers (99+104/2057 to be exact), and measuring with a caliper each spool, and the film thickness, the PICO was able to hold a fairly steady movement for a few dozens of frames without any feedback loop.
You have a better transport setup, tension feedback… but the math says that with large diameters spools, will be challenging to hold precision with the 200 steps/turn of a normal stepper, even if using 1/16th micro-steps (what I had available).
A 50 mm diameter reel translate to about 5 stepper-steps, or 80 micro-steps, per Super-8 frame. Granted, error can be corrected digitally panning from the sprocket reference (when sprockets are available).
After trying, I have a healthy respect for the tolerances involved in the movement of 8mm film, actually seeing some even with a capstan. Godspeed mate.