Xevil still works.

I would be keen to hear back on these. I think you will have no problem with Civilisation, but for Skyrim modding, possibly issues especially if you're using an ENB mod.

I've been meaning to try Skyrim but I'm waiting until total conversion mods Skywind and Skyblivion come out.

The only thing that still makes me keep a Windows partition on my computer is Valorant, I want to start playing CounterStrike so I can drop this damn malware game.

You can just check on protondb if any game you want to play works with Proton or doesn't at all. But generally almost anything that doesn't require an anticheat will work.

I can only speak for Witcher 3 of the games you've listed and it plays really great!

I'd say about 95 percent of my steam collection works with little to no tweaking required these days. Skyrim works, throw any mods you want at it. Witcher 3 runs great. RDR2 and any civ 1-6 all run perfectly. Baulders Gate 3 runs smooth as butter.

There's the odd game here and there that doesn't work well or at all but most things just work. I think the main thing that doesn't work is anticheat enabled online games. I don't play those though so I can't really comment on their state of play outside of that.