I also couldn’t get sddm to work from the KDE settings. I changed the sddm.conf file manually and I’m using the catpuccin theme. The Arch wiki helped (as usual).

I haven’t noticed any issues. But only Plasma would be a DE. Qtile and hyprland are WM, not sure if that makes the difference.
Initially I had doubts if it would work, but so far so good.

Nice work. Why move from plasma to hyprland? Install both and login on the environment you want at any time. I have qtile, plasma and hyprland