I haven't gotten around to testing this yet.
However I do have a new question. In SDDM login page, there is a menu where you choose your session. Only one of the three actually gets you in to a working Xfce4 desktop. So to avoid accidental problems in the future, is there a way to either edit the contents of that menu, or hide the menu, or both?
I think that you want to edit the list of available Desktop Environments when sddm starts, is this correct?
please keep in mind, before we explore this topic, that removing unwanted desktop environments using a package manager is far better than pretending that it isn't there. I recommend pkg remove command
if you really wish to pretend that a DE isn't installed, then deleting the .desktop file in the xsessions folder should remove it from sddm available DEs list. In such a case, i recommend that you copy the .desktop file somewhere on your hdd as a backup (oops i want it back moment), then delete the .desktop file as root or as a user with root privileges (su, doas, sudo).
nota bene: please ignore all of the above if you only have one DE installed. In that case, what in the world have you done to create more than two entries? I hope that you mean default session (X11 with TWM), XFCE Session (X11) and XFCE Session (Wayland). I would not recommend removing default X11 for admin purposes.
.desktop file locations:
/usr/local/share/xsessions
/usr/local/share/wayland-sessions/
example using doas:
doas rm /usr/local/share/wayland-sessions/xfce-wayland.desktop
is this what you want to do? it works for me in Debian (i have Gnome, KDE Plasma and XFCE 4.20 and i removed the xfce-wayland .desktop file).
I hope that this helps you and let us know if you iron out the desktop-installer wrinkles