As far as I understood, currently most if not all plasma services are automatically launched on login, and they are defined in /etc/xdg/autostart/. This has several problems, such as annoying ones like DiscoverNotifier, or security relevant ones like kdeconnect-daemon not being trivial to disable. I am currently experimenting with neutering these autostart entries and replacing them with systemd services. But I am not sure if I know what I am doing, so I am happy for help. Example for KDEConn...