Shim the movable vari cheek so it’s tighter at rest.
This is a common problem on hobbits.
You shim the vari to push it in farther taking up slack so the belt is tight af in the cheeks and you can pedal start.
I doubt it’s the belt angle. Belt angle in my experience isn’t that important, like almost at all.
The test for this is if you can spin the vari by hand at all when the rear pulley is locked. If you can spin the vari you have to shim it or get a wider belt.
On hobbits the shims go between the vari and ramp plate to push the vari in further at rest. The shims float on the boss.
Hope this helps. (edited)