There’s a thread here from 5 months ago with some more info. It helped me, but not enough. After some trial and error I have the server running, but when connecting Remmina I get a black window for a couple of seconds and then it exits/fails.
I see I had working remote desktop back in NixOS 22.11: Remmina client to GNOME remote desktop server (RDP) results in black screen · Issue #238800 · NixOS/nixpkgs · GitHub.