The benefit of the belt is limited, current belt last not as long as older ones.

And belt have a lower efficiency than a chain.

Last point, there are no small cogs for belt like for a chain which is a problem for a small wheel bike.

Tensionning is also a problem, not only for folding, but also because the distance between crankset and rear wheel axle changes when the rear suspension works.

For the hub, Rohloff is excellent and compatible with small wheel. That's what I have on my Brompton with 54x13 it gives about 1.5m-8.0m gear inch with 14% perfectly equal spaced speed. Rohloff also allow to switch speed while pedaling with power which isn't possible with other IGH and isn't too heavy (actually a non disc Rohloff with some small weight savings has the same weight as the Brompton 6s BWR).

I could take a Birdy with Rohloff but to minimize the weight, I preferred a derailleur that with my current setup 50t front 9-34t rear provides a enough range.