Playing with #greetd at the moment, and #ReGreet with cage is a no go, it either opens on the last output, or spans both, while I want it to open on the first. I can fix that, though.
Not a fan of #tuigreet, so that one's out.
The bigger problem is that... greetd does something differently compared to gdm. After logging in, things start up incredibly slow. I suppose that's some dbus thing not starting or something.
(Also, all greeters list every session twice for some odd reason)