Input not working comes from people not installing the steam-devices package, it doesn't matter if you are using the Flatpak or the repo version (for example the magnificent ublue distro comes with that rules added by default). For using steam you don't need to change any permissions on the Flatpak. And about the drivers, you can use the Mesa beta channel Flatpak if you want to have the latest.

Also, the Flatpak works perfectly with mangohud, vkbasalt, gamescope plus everything is containerised and doesn't litter your home folder.

Immutable distros are the future now, and practically excluding them from your guide doesn't makes sense to me.