Don't game much but I'll pitch in with a different suggestion: Debian minimal install or Arch, same as for every use case.

I prefer apt based distros so I always go Debian, but with flatpak it's basically irrelevant afaik.

I've had no trouble mimicking my deck's setup and game compatibility on basically any distro, use gamescope, proton etc.

Debian isn't bleeding edge but it also just werks and with flatpak I haven't encountered any issues getting new stuff essentially totally segregated from the main OS then nuking it if anything is borked without reinstalls or any downtime.