I can't imagine why you wouldn't directly couple the film sprockets to the pan rotation.
Third, the way I have it set up now, the speed of the winding motor actually has to change as the camera pans because the effective diameter of the takeup spool
Because it would be difficult; it would amount to making my own camera from scratch
So that when you advance to the next frame, it goes exactly 8 perforations, and not too far (wasting film) or too short (overlapping images).
you are using any kind of conventional camera body, it MUST have at least one proper sprocket that "meters" the film.
Not clear what is the benefit of using old-fashioned chemical film? That is a dying (nearly dead?) 19th century technology.
I cringe at the thought of using "rim drive" for something like your application.
the camera has a mechanism designed specifically to draw the film with the correct tension and at a steady rate