Except SteamOS is also just "a linux box running steam"... A PC with a layer on top. The only differences I'm hearing you wanting is immutability, and discrete version updates instead of a bunch of package upgrades. There are several immutable distros, and updates can be made painless in a variety of ways.
The SteamOS UI is big picture mode now. Since some update or other, the old big picture mode from SteamOS 1.0 got updated to the SteamDeck UI for desktop as well.