Did you try putting in neutral to start. I have had a few times in park it did not want to turn over. Like the machine thought it was in gear. But fired up fine in neautral. Just a idea..
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Mine does the same thing..... Step on the brake and it will crank.

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.

Smoking the belt ensures it's now not only glazed and has burned compounds, but will start coming apart right at the worst time. Do yourself a favor and change it before the trip and use the old one as a spare after scotch-briting the glazed areas.

Have you noticed a new vibration?  On a snowmobile, smoking the belt will often give it an "hour glass" where the primary sheave closed and slipped on the belt.  The hour glass will cause vibration and is tough on the sheaves.  When you have the original belt in hand, pull it between your thumb and index finger. If it is hour glass'd, don't even keep it for a spare.

When I bought the 800 it came with the kwt installed.  Dealer said they were not interested in installing the rear window do to having to cut holes for the kwt mount, and that was okay with me. If you want it done right do it yourself.  So left to my own ideas and tools this is the result.  Before and after pics....