You’re supposed to put some type of desiccant like silica gel beads that will pull that moisture out of the air.
Leaving the box open will just let humid air in. You can’t let air out without also letting air in unless you have some sort of vacuum pump.
Not one of the types you were asking about, but Hades feels like it’s made for the Steam Deck, and it’s a great game (and also on sale)
For FPS games, Doom (2016) is one of the all-time single-player FPS games and runs great on the Deck.