Nice and clean solution! One thing though, instead of creating a new service to do the job, just add the script to /etc/systemd/system/octoprint.service, as ExecStartPre and ExecStopPost:
...
ExecStartPre=/usr/bin/octoprint start
ExecStart=/home/pi/oprint/bin/octoprint serve --host=${HOST} --port=${PORT}
ExecStopPost=/usr/bin/octoprint stop
...
This way, you sync the settings on each octoprint service restart.