Note
Look, I'm no linux expert. There's a good chance I probably did more than was necessary here, but this is what worked for me, and I need to remember what I did so I can undo it later. If you cook your PC, that's on you for following the instructions of an random idiot on the internet.
After some looking around on protondb, it became pretty clear that the reason I couldn't play Ghost of Tsushima was because it was borked on the 580 nvidia drivers. After some experimentation, the below is what I found to work for me, and got the game running flawlessly. I couldn't find any straightforward instructions anywhere that explained how this process should work, so I had to cobble it together myself.
Maybe this will help someone else, and if you can see anything in here that oculd be improved please leave a comment!
To begin with, I was on version 580.82.09 of the nvidia drivers, and 6.17.1.arch1. Steam was installed via flatpak, hence the couple of extra steps at the end.