There's currently somewhere around 68,000 games on Steam and so it's going to take a long time for Valve to check them all on Steam Deck but here's some recent picks.
I guess if everything just worked, that would be fine, but as of now they don't have drivers for wireless Xbox controllers, so you are stuck installing them manually after each software update. Plenty of software just plain isn't available via flatpak, so you have to reinstall it every software update as well, since anything you do via package manager gets deleted each time it updates.
I just don't think they can predict everything that someone might want to do, which is why additional software exists.
Well SteamOS is Arch based but running a LTS kernel with backported changes in a ummutable way with everything sandboxed in Flatpak so it's quite unique but idk why anyone would want to run it on their desktop, if the immutable aspect is so interesting ro you you can try Fedora Silverblue, Vanilla OS and co. but none of those is in a state that I would recommend as "just works" for a desktop experience, if that's the majore goal Debian or Fedora with Gnome are probably your best options.