Photos Library not showing up when syncing iPad or when inserting media into document/presentation/iMovie etc.

I have recently updated to MacOS Catalina (10.15.7) on my Late-2012 iMac 27".

I have also recently purchased an iPad Air 4.

All my software is completely up to date (as of 3rd Dec 2020)

It seems to me that the media browser built into Catalina isn't able to see my Photos.app library, even though the library opens perfectly in Photos and I can see all 20,015 photos and 705 videos just fine, along with several albums that I have set up. The music and movies tabs in the media browser also come up empty even though the Music app has my entire music library in it and it also opens just fine.

Everywhere I have looked online suggests that I need to set my photos library as the "System Photo Library" but I have already done this and it hasn't worked. I have tried making a 2nd test library and switching to that (and switching back again), and it is still not recognised by the Sync app or by the Media library in apps like Keynote, Pages, iMovie etc.

When I try to sync photos from my Photos.app library to the iPad over USB, the library is not visible and I can't see any of my albums etc. If I click "Sync", it first backs up the iPad and then I get an error message saying:

"Photos cannot be synced to the iPad “Guy's iPad” because your Photos Library could not be found. Open Photos to create or update your Photos Library."

I have spent hours searching the internet for a solution to this problem and have come up empty. It is very frustrating.

Any ideas?