You can move the PMS_APP_SUPP_DIR to the external drive by using the systemd override.conf mechanism.
The symlink is not 100% safe/reliable. Override.conf is best.
You can customize a few other things about it too.
Take a look here
Example. (My /usb is an external M.2 SSD mounted as “/usb”. )
[chuck@lizum ~.2002]$ cat /etc/systemd/system/plexmediaserver.service.d/override.conf
[Service]
Environment="PLEX_MEDIA_SERVER_APPLICATION_SUPPORT_DIR=/usb/plex"
User=chuck
Group=chuck
[chuck@lizum ~.2003]$
I stopped plex, Copied the data (as root) using tar | tar then a chown after the fact. Done 
