I meant the:

-force-d3d12

one. Might need to put "%command%" after it, but I'm still not 100% sure what that even means and why its there so I dunno.

Edit: I would also check out the entry for the game on ProtonDB if you haven't already (sorry if someone else has already told you all this elsewhere in the thread) to see if anyone else has had similar issues: https://www.protondb.com/app/949230

I haven't played C:S2 yet, so I can't say personally, but it could just be a poorly optimized game.