A lot of older PC games can run just fine on modern phone hardware. I’d buy a SteamOS version of a phone that has some modular or built-in set of buttons and analog sticks. I don’t know how the app ecosystem would work for sensitive things like banking but it’s mostly a minor issue for me.
Is Valve’s update cycle really worse than what the typical Android device gets?
No but also yes. No because Valve supports their hardware extremely long but also yes because several design decisions of SteamOS as seen on Steam Deck weren’t made with data security in mind. Storage isn’t encrypted, Game Mode has only a simple PIN lock but the underlying Linux account “deck” has no password, so Desktop Mode (=KDE Plasma) cannot be locked out of the box.
That said, Valve will release a Linux ARM version of Steam later this year, so there is no need to solely rely on Valve for a Linux phone that runs Steam and its games.