If you want SteamOS, your best bet is still Steam Deck. If you don't mind using Windows on a device that's not built for it (which does come with the benefit of compatibility and Game Pass), that's where most of these other options are going. I hear good things about the ROG Ally; seems like that's the go-to if Steam Deck doesn't get the job done for you. If you want some thorough reviews to help you out, check out The Phawx on YouTube. That guy has been doing thorough reviews of handheld gaming PCs for a long time now, and you'll be able to figure out which one is the one for you.
Ganondorf