Your question implies you want to put the Plex metadata on the QNAP. Putting the Plex “Library” on any NAS requires advanced skills and, it’s not something which can be supported. If, mostly when, the data becomes corrupted, it’ll likely require a full rebuild of it all which makes the entire purpose moot.
It is very easy to move the data out of /var/lib/plexmediaserver to another location on the Linux system where PMS is running.
Any internal or external, ext4 formatted, drive will do.
The procedure is here.
