This description of the initial installation is correct 
However, your first post shows, that there is a systemd drop-in in place: /etc/systemd/system/plexmediaserver.service.d/override.conf What is in there and why do you need it?
Please make sure to set the exec mount option for this network share on your Pi. Plex needs it and crashes without it. See for example: Plex Conversion failed. The transcoder exited due to an error
If your server is still crashing, please also attach server logs.
https://support.plex.tv/articles/200250417-plex-media-server-log-files/
