I never needed my computer for any reason while setting either of my Decks up so you should be good. Never tried any Rockstar game on the deck so I can’t be sure but I don’t see any reason a computer would be needed.

You can always just switch to desktop mode on the Deck and use the regular version of Steam if anything requires it. The only time I’ve needed to do that though was when I wanted to add some box art to a non-Steam game.

Just make sure all your cloud saves are synced from your PC and you should be good!