Afaik SteamOS still only supports very limited hardware configurations similar to the steam deck, for example only AMD GPU are supported (Nvidia is in beta support as of recently, I think?).
Personally I can't run steam and a game on a my laptops. They're good enough to run games like subnautica and stalker on wine but steam requires like 1gb of RAM and runs like shit.
SteamOS 3 is Steam big picture mode inside of gamescope (a standalone wayland mini compositor), without your KDE desktop running in the background. You still have bluetooth, wifi and whatever other background processes running, but if you want to use a video editor, use your terminal or something else not on Steam then you have the option to boot out of gamescope session and into desktop mode from the 'Power' menu option. In game mode you still have to deal with Steam being a webapp but with no other desktop programs running, aside from the game you're playing.
Wouldn't it be the opposite if SteamOS is lighter? I'm already running things like i3 but having even more stripped out sounds like it might be something. Maybe I'm misunderstanding