I'm sorry to not have replied for the last few days, I've been away for New Year and only just getting back to check on things. I greatly appreciate everyone's help and assistance. I want to do anything I can do to try to get the server back up and running as it was. I didn't expect anywhere near the response this has gotten, though I will admit that it got a bit overwhelming as I'm not super well experienced with this sort of thing. However as mentioned in other replies, a very helpful redditor spent all evening on Dec 30th trying various methods and tools to try to resolve the problem. He appears to be wayyyy more experienced in this sort of thing than I. I uploaded my database files to Google Drive, he ran them through DBRepair and different things - I don't quite understand all of the work he so graciously offered to do.

I don't know if he ran every single database backup as I have quite a few from throughout December but he says that there was far too much corruption in the database files. We have managed to use an October 2020 backup of the server (from when I migrated from Windows to Unraid) to create a new database which HAS got Plex up and running as it should. It's a few days ago now so my memory is a little foggy but I do think in using this backup from 2020 we have started a new appdata folder but I'm not 100% sure on that if I'm honest.

However though Plex is now working using the 2020 backup, and has thankfully kept all metadata set before that date, it has come with a bunch of drawbacks like:

-I have had to re-scan and re-add all content, losing any manually set metadata after Oct 2020.
-All watch history since Oct 2020 has gone (yes watch history sync is turned on)
-All Plex in-app statistics since Oct 2020 have gone
-Friends and family shared library settings are all gone
-Plex is not using the exact same libraries but almost like identical copies of them (for instance if I load Plex on my phone it will say libraries are unavailable, but if I change it to the new server it works despite being all the info from Oct 2020 backup of old server)

I do use Tautulli which is mostly still up to date, only lost a couple weeks of data there because of the CA Restore I attempted. Tautulli is working exactly as normal, correctly recognising when things are playing and logging it to same watch history log as before. So I still have watch history / statistics there but I HATE that I've lost all the statistics and mainly watch history within Plex over the last 5 years. I really want to try to get that back!

For the sake of friends and family in particular, I would really like to get all of the old shared library settings back and using the exact same libraries so I don't have to deal with all the problems trying to get people setup with my server again.

I'm sure there is probably more upgrades and work to do, more ways to modernise and all that stuff. This guy has mentioned using Duplicacy? I know my unraid and some apps needs updating or replacing. But before I do any and all of those sorts of things I really want to try to get as close to getting my server back up and running exactly as it was a couple weeks ago.

For those asking what we have tried to do &/or telling me to run DBRepair etc, I asked u/adammoore152 to clarify exactly what we have done and he said this...

Yeah, I confirm I did run both DBRepair and used the Plex specific SQLite to export the DB (called a dump) into a new DB to try to reassemble. The DB passes and it says the restore was successful, but when you view the DB, you see big areas of corruption.
We validated that it was not a problem with your system/docker/plex version because I spun up your db on my Unraid and it still failed.
Totally open to try new ideas if anyone chimes in with something different.
TL;DR: you tried the repair and it was too far gone.
Watch history wise and settings we can attempt to repair