Technically speaking, Steam handles that part automatically. You wouldn’t need to futz around with it in any distro so long as steam is up and running. That said my original idea that you could just launch steam from an Xserver login script is, well I’ve since learned that Steam Decks are running a less than simple setup behind the scenes. BUT from an end user experience, booting any old distro straight into steam big picture should be at least a passable Steam OS experience, barring any performance issues that would result from the difference in back end implementation.