If the service is already running it has to be stopped as a system service and run as a user service. In order to ensure that the service inherits all the correct permissions / acls / se linux policies the service needs to be launched from the limited permissions context.
With the systemd approach you’re not just passing a control handle around. You’re ensuring the process is running under an appropriate security context.
If you want to let multiple users manage the user systems service, I would probably go with sudo and systemd user files. You could create a group which has sudo access etc. The important idea is that an unprivileged user controls an unprivileged service.