I think the tension units are a great design. Matched with a good motor and some electronics, you will be able to control the tension of the film quite nicely. Depending on the software, I think it should be even possible to adapt the tension for fragile films.
Decoupling reel axis and motor via a belt drive will allow other builders to try out and use alternative motors - a specific motor model might not be available or difficult to source in other parts of the world.
The same point can be made for the more complicated gear options you mentioned in the video - that specifc planetary gear or worm drive migth be very difficult to source at some places around the world. If you stick for the design to stuff which comes out of a massmarket product (scooter, 3D-printers, etc), chances are that it can be sourced for a resonable price at many places.
From my experience with old Super-8 projectors which did use a friction drive, I would suggest to stay away from that idea: it’s a common point of failure of these projectors after some usage. The surfaces which define the friction drive deteriorate over time or at least change their properties during usage; you might need to adjust them after some heavy use.