I believe, ultimately, the most frictionless solution is going to be to run both Plex Media Server and Kodi under the same account (likely your own). It’s not the only way to tackle this, but will likely require the least futzing about with ownership and permissions on your part.
It’s relatively straightforward to change the user for PMS; a systemctl edit plexmediaserver should get you started. Of course after that you’ll need to chown -R /var/lib/plexmediaserver and any other locations which PMS cares about. For Kodi, there’s likely a similar procedure.
