It would be great if SDDM could run under wayland. Documentation of SDDM says, that SDDM is abble to run under wayland. It's a little inappropriate to run Xorg to run wayland session. It would be great if x11-misc/sddm had USE flag, which can change default configuration to run SDDM under (for example) kwin compositor. If this is not possible, it would be a good idea to run SDDM under X11 without root privileges. This way is for sure stable.
It is experimental and broken for every second user trying to use it.
https://github.com/gentoo/qt/pull/267 was where we did some WIP on it, just for completeness.