I just want to get this in so we know it's been tried and seems to be an improvement over the version in the first video:

The proof of concept constrained the timing belt along one edge to a circle. When the wave generator moves past a section of the belt, it caused deformation across the belt in a way I don't think the belts are designed for. I have printed a second version of the gear where the belt is not fixed on one side. Instead, it has a second "outer" toothed ring. Now, we have one ring with N teeth (60 in this case) and one with N+2 teeth (62 in this case). The wave generator arm now pushes the belt into the oval shape evenly across the width of the belt and the gearing effect is still achieved.

I'll try and put something visual on here soon to explain it better.

Onwards to the bearing phase! I have some 608 skateboard bearings left, a 6" lazy susan bearing and some wider diameter ~40mm deep groove bearings. Wondering if a couple of angular bearings are a good way forward. Thinking the 608 and a larger ~40mm dia bearing mounted concentrically would be a good way forward. Wave generator in the centre as before and the two outer gears on the middle and outer races/rings.