Using WantedBy=multi-user.target might work to start the unit at
boot-up, but the unit will not be started in other contexts such as
"systemctl restart user@<id>.service" which is useful in to restart
user services identified by needrestart.

From https://wiki.archlinux.org/title/systemd/User#How_it_works :

> When a systemd user instance starts, it brings up the per user target
default.target.