The cache will only boost repetitious data transfers. You would have a better Plex performance if you had one or two SATA SSD in HDD bays. This is where you would install DSM and apps including Plex.
Qnap NAS allow installing OS and app on NVMe volumes.
I hope this helps.
Qnap NAS allow installing OS and app on NVMe volumes.
I hope this helps.