don't use the old steamOS, it's very out of date and deprecated. i'd say to use arch linux or something along these lines to recreate the Steam Deck environment
this is gonna be good for most games. just make sure that if you require a wireless connection (wi-fi) then you will need a wireless card. i would go for the intel wi-fi 6 ax200.
i'd also recommend for getting the arctic thermal cooling solution that they offer if you are going to stick with a...
the inwin 4u rack chasis has bays for 4 3.5" and 1 2.5" drives. your best bet would be more along the lines of a server case. if pcs doesnt carry a specific case, you can technically send in your own, but obviously the cost of the case and the courier would be on you.
as for the motherboards...
i would LOVE if pcs offered like a mechanical keyboard building service, because as far as i'm aware there are very few, if any, mechanical keyboards that they stock
