I think the big takeaway here is the downgrading of the Nvidia drivers. I am also running Cinnamon, but since switching to the v470 driver branch, I have had zero problems with Steam, including games run with Proton.
I really don’t think that Cinnamon is the issue here.
Since I downgraded the Nvidia drivers before the Cinnamon 5.2 upgrade, I can’t simulate your environment. While Cinnamon might be part of the problem, I still believe the v495 driver is the bulk of it. Before I dumped my dual-boot, I was running the v496 driver on Windows 10 and all of my Steam games were laggy as hell.
Remember - the v470 branch is the production driver, while the v496 is the new-features one.