You can see it looks for a script to shutdown steam or defaults to normal shutdown. I pointed os-session-select to a script that restarts my sddm service, before shutting down steam, so it returns me into the default session. It was a bit finicky though and I hacked a systemd service into it to ensure the script didn’t get killed.
Hope this helps. Might clean it up some time and put it in a repository/on the aur.
EDIT: I was inspired by ChimeraOS; it uses that os-session-select for its main project as well to return to the gnome desktop.