If you are willing to ride like a scooter with almost no pedaling, you can stay with the power of 48V and get LightningRods 18T slip-on pulley. That will provide a significant increase in belt-holding (20% more teeth?). If that is not enough (belt still skipping/ratcheting) then perhaps the next step would be buying the second large pulley, and when you have the two-pulley upgrade it would allow the use of the wider 20mm belt for 30% better holding, plus the 20% for more teeth on the motor-pulley.
Belts are quieter, especially on the high-RPM primary. If you don't mind the extra drill-like noise, chain is cheaper and it will take the most power while guaranteeing no slippage. #25H chain will work fine (H means heavy-duty). The pain of a chain-primary is turning down the motor shaft to an accurate and perfectly cylindrical shape (Denisesewa showed how in the linked threads). A 10mm shaft can use existing #25 spockets, and if turned to 12mm, LightningRods will be stocking an assortment of pulleys and sprockets, but...you would have to wait for those.
