I use SDDM and it does appear in the drop down menu next to the Wayland option. Also can be set in it's config file. So ive been thinking about using a script that uses systemd's soft-reboot and edits that config for quick swapping. Apparently the "Switch to Desktop" in the steamdeck ui is scriptable aswell to go back.

In arch's AUR there is also a neat package i've been using that I think is worth mentioning called gamescope-session that's from chimeraOS. It skips loading a DE and just runs gamescope and launches steam in the steamdeck UI.