In KDE/plasma5: click system settings → in the list which appears there is no choice of connection screen (SDDM) as in the other distributions.
I have activated in the configuration file the following
# Enable the Plasma 5 Desktop Environment.
services.xserver.displayManager.sddm.enable = true;
services.xserver.desktopManager.plasma5.enable = true;
Do you think that it is possible to do something to make this choice appear?