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.
Note
The aim was to downgrade to 570.153.02-1 of the nvidia drivers, based on what worked for some folks over at protondb.
- Look at releases of the nvidia drivers here to work out what version of the kernel I needed.
 - Cool, I need version 
6.14.7.arch1-1for version570.153.02-1of the driver. sudo downgrade linux linux-api-headers linux-headersand select that version from the list for all three- Add packages to pacman ignore list when prompted
 - Reboot the PC.
 - Open a terminal in potato resolution because the drivers can't load
 sudo downgrade lib32-nvidia-utils lib32-opencl-nvidia libxnvctrl nvidia-open-dkms nvidia-settings nvidia-utils opencl-nvidia(basically anything with the 580 version number mentioned earlier)- Add packages to pacman ignore list when prompted
 - Reboot the PC
 - The PC should boot up in a normal resolution with everything working, and when running 
pacman -Qs nvidiayou should only see the 570 versions. - (Note the next few steps are only relevant if you have installed steam via flatpak).
 flatpak install nvidia-570-153-02will give you two options. I ended up installing both of them, and that worked, but I doubt I needed both.- Your ghosts should now be tsushiming
 
- Remove the ignored packages from 
/etc/pacman.conf sudo pacman -Syu- Reboot
 - If the drivers don't come back online, 
nvidia-inst, then reboot once more. - It's possible that the drivers you have now are newer than the ones you downgraded from. In which case, do a 
flatpak updateto pull in the new drivers for steam.