[Melcar]

Ubuntu or it's derivatives. With a careful use of PPAs you can have a very good and capable system. Canonical has been doing some stupid stuff lately, but no big deal. Since you are using nvidia, I would avoid rolling release distros like Arch since those can be a nightmare with proprietary binary modules.
Steam will work no problems and most games can be made to work with Proton. Other launchers you can easily get them to work with Lutris. Anti-cheat will be your biggest problem without resorting to some complex hacks, but there is some work being done in that area from Valve and Epic.

 

[Melcar]

I largely agree, although to be fair, I've had less trouble with nVIDIA drivers running Arch than with Ubuntu derivatives - they're well behaved until you have a big update (like moving from 18.04 to 20.04, or 20.04 to 20.10, or Mint 20 to 20.1) then things tend to go to hell in a handbasket. I run a couple of Arch systems at work and they're always well behaved. The only times I've had major issues with Arch is when I've not updated it for a very long time (I'm talking 9+ months here, because I stuffed a laptop in a cupboard and ignored it for nearly a year).

But the potential for things to go wrong with a rolling release distro are higher than one with a more stable base. Given that the Steam Deck will apparently use Arch, Valve must be fairly confident they can make it more approachable for non-technical users...

Valve will probably curate updates. Sort of like Manjaro. I doubt they will just latch on to Arch repos and update their devices just like that. It will be an Arch based distro maintained by Valve.

Arch really is not that difficult a distro. The only major hurdle for newcomers will be the initial setup. Once that is done you only have to learn some pacman syntax. Both can be alleviated by using any of the Arch distros that have installers and use graphical package managers like pamac or octopi. The biggest issue I see is users not being responsible enough not to do dumb stuff like unattended updates and/or liberal use of the AUR.