Supraland is the game you want - it fits all your criteria except the most important one: it’s currently Unsupported on SteamDeck. I found reports of people getting it to work but not without some fiddling. It has a sequel, though, which is just as good and which is Verified. The sequel is called Supraland: Six Inches Under.

The first game is an indie gem made mostly by one guy but it is very polished. This sequel was made by a small team. Don’t let the cute graphics throw you - it’s not a kids game. It’s a first-person open world metroidvania (not really a shooter, though there are enemies you’ll be hitting) with gameplay reminiscent of Portal, Zelda, and Metroid. Currently on sale for 40% off ($11.99). About 8-20 hours long.