I don't know if this applies to the 2021 ZForce 800EX, but on my 2023 ZForce 800 the minimum belt width shown in the service manual is 33.5mm. The Belt is equivalent to an Ultimax UA485, so you might find the specs on it to see what the original width is and how much wear is permitted before replacement is needed.
As for the jerky starts and shiny belt, I can relate. On my last trip out my GF put mine in a ditch and tried to power her way out, generating some belt smoke before I could stop her. Since then mine has been jerky on starts. I haven't examined the belt or replaced it yet though. I did have it dealer serviced after that trip and they didn't mention anything about belt wear or possible replacement, although they found a few other minor items to fix. From what I've found, in many cases the wear on the belt from abuse like this often diminishes with a bit more use and wear. I'm going to let mine go for at least one other week long UTV trip and see how it goes. If it doesn't improve a bit I'll probably replace my belt.