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.

pretty sure the runnit solution won’t allow this either

I'm no expert, but I think you could make a special group, set the supervise directory to be owned by that group, and add all relevant users to that group? Either way, as I explained in a different reply, running the service as a user vs letting that user control a root service are completely different things, and one is not always a substitute for the other.

A generic stack overflow answer:

Do you REALLY need multiple users to manage that service? Maybe it's better to have multiple instances of that service and.... (This goes on and on)

he don't want to log as root, so a root shell isn't what he want

Ok, just stop complaining. Almost everyone else disagrees and most of the community doesn't even know that there is a different init system. Systemd was widely accepted 6 years ago and we have moved on.

The good news is that you don't have to use it. The bad news is pretty much everyone expects you to be using it.

If it’s providing you the functionality you want using an overhead you’re fine with, there’s no reason to change it.

In my personal opinion, correct me if I'm wrong:

Systemd was created to replace the init system, then through extreme scope creep took over way more than wanted and needed, the main developer was "problematic" to say it politically correct, and in practice it has over complicated many super easy tasks to the point that I hate it. Other init systems were intuitive, systemd is all but.

Few weeks ago I setup a systemd server ssh server. Changing the port would be 5 seconds in changing a line in the sshd config, but now with the new and improved systemd I need to follow some nightmare documentation into creating systemd files in unrelated places and reload configs or something and I'm done with it

Mint 21.3 is based on Debian Bookworm (via Ubuntu 22.04, not counting LMDE of course). I don't know what you're looking at and I also don't fully know how this works, but what you said doesn't seem to be the case.