Not quite all belt- the first stage is gears.
In any case, this is a very interesting concept. I really like your compact execution of the idea.
The one thing that I'm worried about
: why 9mm wide 3mm pitch GT3 over 15mm wide 5mm pitch? Even using 2 parallel 9mm belts (which may face issues with not lining up), this looks like it may not be strong enough.
I thought this type of gearbox would be unworkably large, but the first stage gear takes up most of the reduction so it works out.
The flipped cylinder arrangement looks a lot like 192's gearbox this year.
What are the reduction ratios?