Hi thank you for your reply.
I already have MariDB installed on my current NAS to synchronise my metadata and watched status across my numerous Kodi install.
The plan for the RPi-4B was to boot from the microsd card I have coreELEC installed on (SanDisk Extreme Pro 128GB microSDXC Memory Card + SD Adapter with A2 App Performance + Rescue Pro Deluxe 170MB/s Class 10, UHS-I, U3, V30) with write speed of 90mb/s. The SSD now are fairly cheap any way so I could always upgrade moving forward if the need is there.
The only benefit I see with Plex is that I can install Plex app directly onto the TV (no need to buy further hardware) or onto the computer.
With my Wetek Hub no longer supported moving forward, I have either the choice to keep Kodi 18 across as I have that shared database. Or I have to purchase a new box for the living room to replace my Wetek Hub and another one for the bedroom. And that leave me with my old current NAS that isn't powerful enough to run Owncloud.