If you really don't want a immutable distro, you could use Nobara. It's based on Fedora and lets you install/remove packages as needed. It is optimized for gaming and has a Steam Deck version available.
That being said, I think Bazzite would be a better option. It is immutable like SteamOS, but it also bundles a bunch of other options like Flatpak, Homebrew, and Distrobox.