I can say garuda linux (KDE Dragonized gaming edition) myself if you want to give that a shot. I did swap from windows 11 to that after some testing with other distros it was the one that felt like it just worked out of the box. Unless the game you want to play runs some form of anticheat it will typically work.
I did also get CnC3 working on it through steam/proton. As for how fiddly it is to get games running. If you own them on steam you pretty much just need to go into the properties and flip them over to use whatever the latest proton version is and install as normal. Modding will take a few more steps when it comes to skyrim etc but i havent really tried going into that too heavily myself. Unfortunately the vortex mod manager pretty much explodes if you try to use it on linux so you end up having to install mods manually but there is a mod manager that may do the trick “Mod organizer 2” but I’ve never used it.
As far as i know most MMOs don’t run heavy anti cheat stuff as they are built to not trust the clients so i suspect a lot will work fine.
Some i know work perfectly fine are:
Guild wars 2, both through lutris and proton. Back when i originally swapped to linux i had some issues getting the commonly used dps meter (arcdps) to work with proton. It requires some optional features to be enabled to allow direct x overlay windows etc. Worked first try through lutris though and ive been playing the game on and off for the past year through that without issues.
The Elder Scrolls Online, works perfectly fine through proton. You can even get the standard addon manager (minion) working through standard wine and just point it at the right folder for addons to work. A few that require running programs to update stuff will be fiddly though. (Like tamriel trade centre)
Final fantasy XIV has an unofficial launcher (XIVlauncher) you can snag up on the flathub that makes installing and running the game easy. I have not played the steam version, but i imagine it works as well.
Warframe works fine through steam, but its barely an MMO.
Second life works fine with the common viewer, firestorm, having a native linux version.