Server Version#: 4.63.0
Ubuntu 18.04.6
Regarding @ChuckPa 's excellent turorial here, I found it necessary to escape the space in the path while creating the override file, like this:
Environment="PLEX_MEDIA_SERVER_APPLICATION_SUPPORT_DIR=/mnt/bigdisk/plexdata/Library/Application\ Support"
Without the backslash the service failed to start.