I have Xmonad running well on Ubuntu 24 with Gnome Flashback

The packages I install are: xmonad libghc-xmonad-extras-dev gnome-flashback gnome-panel

(plus suckless-tools and xmobar)

That should give you a login option for "GNOME Flashback (Xmonad)"

I recall there were a couple of hacks necessary to show the Gnome Panel:

gsettings set org.gnome.gnome-flashback root-background true

gsettings set org.gnome.gnome-flashback desktop false

and then the panels you can hide or remove per your preference