As I said before, the above script runs the service as the configured your (eg: your user, not root), even if it’s started by root (systemd will change user before starting the process). (I’m assuming that’s what you refer to with “user space”, since syncthing can’t run outside user space).
I do agree on the multi-user point; syncthing may degrade performance on shared hosts or have other undesired effects. Just also keep in mind that by running with systemd --user syncthing will not sync files until you log in on more than one host simultaneously.