Seagate Personal Cloud - upgrade HDD?

[shed driver]

I've got a 4Tb Seagate Personal Cloud - model SRN21C that's no longer supported by Seagate. It's running a Plex server and has my collection of films (approx 3.3Tb) on the local storage. It's connected via a Roku 4k to the TV and for the last 6 or 7 years it has run faultlessly.

I can back up the media via the USB 3 connector to an external HDD. I'd like to put a newer and larger HDD into the box and keep the system "as is". It's very quiet, unobtrusive and more importantly my partner can use it!

Is it just as simple as swapping drives? I had heard the OS was on the HDD so this might complicate things.

Otherwise, is the a similar NAS with a small form factor that can run a PLEX server and has more storage capacity? Form factor is important as SWMBO won't appreciate a huge 8 bay NAS under the telly!

Link to user manual https://www.seagate.com/files/www-content/manuals/...

SD.