Steam is natively installed and works perfectly. You can install other packages, but it effectively adds another layer which slows down updates a bit.
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Preface: I don't have any experience with Garuda or Nobara but I have used Bazzite.
Not to make the choice harder, but Bazzite does come with Plasma 6. You can have it boot to the SteamOS UI or to the Plasma Desktop.
Bazzite is a great choice for stability but you need to be aware it doesn't operate like a traditional Linux distribution since it's based on ostree and is immutable. Package installations are primarily done through Flatpak, AppImage or exported via Distrobox.
You might have some luck with Bubblejail or Firejail. Alternatively, you might want to give one of the Universal Blue images a try. They're Fedora based but immutable. Almost all installations are purposely done in a container using flatpak or distrobox.
I found out about OpenLinkHub recently and it works great with my Corsair AIO cooler that OpenRGB doesn't support.
The AUR requires an AUR helper like paru or yay to interact with it that way. Otherwise you can clone the repo using git and make the package yourself.
Is... Is that a garbage bag vest?