I was surprised, in a bad way, at how difficult it is to get any VNC running. I tried Ubuntu, Kubuntu, and base Debian, but couldn't get any VNC working. The closest I got was with Debian, but it gave me a different desktop than what was coming out the video port to my monitor. I'd be interested in hearing if anyone has had better luck with anything.
x11vnc works a dream once you have a systemd service running it on boot, but that rules Wayland out.
You may be able to get similar results by explicitly instructing the others to share display :0, otherwise they default to starting new sessions.