I’ve told you your issues.
Get a stronger spring (1500-2000 spring) and a launch lever, then retune. You could try to add an extra spring but you don’t have the room so GET A STRONGER SPRING LOL. I’m using a stronger spring than you are with my 50cc moby.
The belt’s side angles not fitting well is because er3 has steeper cheeks than most standard V belts to allow more variation at fully closed. They tend to eat belts, especially when you don’t have enough tension on the belt due to a weak spring.
French style bikes want to get as much variation as possible, and often cheek angles vs belt angles don’t match to achieve that end goal. With a baby weak spring those few mm of belt will slip like crazy. With a stronger spring it will grab. Slip creates heat. With a launch lever you can manually add more tension without a stronger spring.
Got it?