I believe wayland.enable enables running sddm itself with wayland.
It sounds like you are trying to set the default session. Does sddm support that? I don’t see anything like that in sddm’s options. You can set the session for autologin but that doesn’t sound like what you are looking for.
services.xserver.displayManager.defaultSession = "plasmawayland"; # for Plasma 5
services.xserver.displayManager.defaultSession = "plasma"; # for Plasma 6 (this is pointless, as Wayland is already the default)