A working solution for KDE has been found (xrdp server missing KDE option · Issue #57717 · NixOS/nixpkgs · GitHub):
services.xserver.enable = true;
services.xserver.displayManager.sddm.enable = true;
services.xserver.desktopManager.plasma5.enable = true;
services.xrdp.enable = true;
services.xrdp.defaultWindowManager = "startplasma-x11";
networking.firewall.allowedTCPPorts = [ 3389 ];
Make sure to reboot after rebuilding.