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. ...
For your unit files, you have Wants in the [Install] section. That is not correct. Wants belong in the [Unit] section. The [Install] section is where you define WantedBys. You may want to read the man page for systemd.unit.
To interact with user services, you do have to always use systemctl --user.
If you put your user unit files in /etc/systemd/user, they're accessible to all users. If a particular user wants to enable the service, they can run systemctl --user enable $service. Defining the unit in ~/.config/systemd will mean only the one user will be able to start the service. Defining the unit in /etc/systemd/system indicates it is not a user service but a system service.
Every user can enable services from /etc/systemd/user for their account. If the user doesn't log in, their instance of the service won't start. There is a way to have user services launch without logging in, but that would obviously be nonsensical for desktop services.
I don't think systemd would find units in /etc/systemd/user/KDE. Look at the mess that is /usr/lib/systemd/system. Organization doesn't seem to be a thing.