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!
Oh and make sure you can access your email somehow, or Steam Guard on your phone if you have it set up, it’ll probably have to verify your account when you sign in the first time.
I’ve always wondered why they use so many as well! I assume it’s each group of items that a single shopper pulls goes into one bag and they have one shopper per like four aisles or something. I have nothing to base that on though, whatever it is it’s a ridiculous amount of bags.
If you get delivery they will sometimes put three or four bags with a single item each all into another bag so they can carry it easier.