I don’t have experience specifically with the R8000 mechanical derailleur, but I have used a bunch of Shimano SS/short cage derailleurs with 32t cassettes without issue.
I’ve also used a variety of Shimano GS cage derailleurs with 36t cassettes, and most have worked ok by maxing out the b screw. The one that didn’t was a 105 5800 GS on a Specialized Crux. For whatever reason, that combo just didn’t want to play nice.
I’d see if you have any friends with 32t cassettes and ask to borrow their rear wheel for a trial fit. Just see if you can tighten the B screw enough to get it to shift smoothly into the 32 while you’re in the small ring. Once you confirm that, it’s just a matter of ensuring your chain isn’t too short to go big/big.