You’ll first need to have these in your SDDM config.

[General]
DisplayServer=wayland

I’d just have it in a new file called /etc/sddm.conf.d/wayland.conf

If you’re used to configuring your login manager thru KDE’s settings panel, you do not want to add to its config file as it will get overwritten everytime you save your settings.

Next would be your systemd. I personally think this solution is less than elegant, but it seems to be well supported, and more importantly, it works.

You’ll need to go to the SDDM’s systemd service file, located on /lib/systemd/system/sddm.service. Simply uncomment a line with

After=plymouth-stop.service

Best to test by rebooting.

Note:

Failure to do the systemd step would result in the sddm.service failing to run. When this happen, simply switch to a free tty and login. From here, you redo the systemd step. You can also restart the sddm.service (it will work).