Related to #460, it is possible to run USBGuard as an unprivileged (non-root) user even with the -f option & Type=forking in the unit file. To get this to work, I had to:

1. Add a new "usbguard" user and group using systemd-sysusers.
2. Change ownership of "/etc/usbguard/" and "/var/log/usbguard/" to the "usbguard:usbguard" user and group. I may have changed other folders as well but I don't fully remember.
3. Recompile USBGuard to write its PID file to the "/var/run/usbguard/" directory (which can be owned by "usbguard:usbguard") instead of "/var/run/" (which must be owned by root, so writing the PID file as an unprivileged user fails).
4. Update the "PIDFile=/var/run/usbguard/usbguard.pid" option, and add the "User=usbguard", "Group=usbguard", and "SupplementaryGroups=" options to the unit file.

EDIT:
5) Add "CAP_DAC_OVERRIDE" to "AmbientCapabilities=" and "CapabilityBoundingSet=" in the unit file. This is needed so that USBGuard can actually write to the "authorized" properties of the different USB devices under /sys. I think this shouldn't be much of a security risk, since other hardening options in the unit file prevent arbitrary writes to the rest of the system.
6) Add "AmbientCapabilities=CAP_CHOWN CAP_FOWNER CAP_AUDIT_WRITE CAP_DAC_OVERRIDE" if you want to allow non-root users to manage USBGuard through the IPC interface.

And after testing, #3 is very obviously not necessary. USBGuard has the "-p" option to specify where the PID file is written to, no recompilation needed. That's what I get for looking at the source code before the man page...

It would be nice if the PID file was in its own folder by default, but it's not a major issue. Given that CAP_DAC_OVERRIDE is needed, I won't submit a PR to make running as a dedicated user the default. But for those who are interested, the above should be all that's needed to make it work.