Cover Art won't display on iMac - both Apple Music and Apple TV

OK...

I've seen a LOT of questions of this nature, and even followed instructions on quite a few of them, but I am now officially perplexed by my issue.

I have a mid-2019 iMac 5K running Big Sur 11.2.3.

Some time ago, after upgrading to Big Sur, my Music Library, although still intact, decided to stop displaying cover art.

I have deleted the Music Library (begrudgingly, as all the info regarding dates added to the collection, etc.), was lost. I've changed and rebuilt music libraries, (all my music and movie files are stored on external drives), and even recently, did a complete reinstall of Big Sur, and then a restore from a Time Machine backup.

But my Cover Art still will not display. And I am completely OCD about it - if Apple Music has been unable to find and download cover art, I have found it and added it myself, in the past. Out of tens of thousands of albums and singles, there would be less than 1% without some sort of "art".

If I go in to "Info" on an album, and I have previously added cover art in there, double clicking the art will add it to the main upper playing screen with a tiny icon of art, but the main larger art box will not display anything.

On some of the albums, if I choose Clear downloaded artwork" (even though NOTHING is visible), then art actually DOES appear - but once again, only in the upper screen, as a tiny icon.

I have also tried signing out of my Apple ID, closing music, restarting, and signing in again, as well as selecting "All Songs", and then going in to the Library to "Download Cover Art" - and even though it shows it's going through this process, I still have NO Cover Art whatsoever.

I don't want to have to go to the point of resetting back to Factory Settings and then setting up the iMac as a new machine, as there would be some settings and documents that I probably would not be able to transfer across from the Time Machine backup, without doing a full restore.

I hope I've managed to explain myself OK here... long winded, I know, but.... you'll see screenshots below to give you a further clue on what I mean...

Cheers!

(You can see the tiny art icon up top, but not in the main display)

Feel free to judge my taste in music... LOL

But as you can see, not a single album displays art!!! 😳