User specific daemons

How do I start and maintain a program as a user specific daemon? I
don't mean run a daemon as a specific user. I need one of each daemon
per user.

Under Linux I could create a few $HOME/.config/systemd/daemon.service
files run systemctl --user enable daemon.service and the problem was
solved. The services specifically include tmux, and ssh-agent or
gpg-agent. There were some other services I used this for but those were
more Linux specific and thus less important.

Before I go with the metaphorical hammer meets nail shaped object
approach and port half of SystemD to FreeBSD using Rust what options
should I try first.