Music library empty

Last week I updated my 5K iMac to 10.15.4 and found my Music library empty apart from a handful of iTunes store downloads from many years ago. I assumed this was to do with the update and so restored the computer from the backup I had made before updating, but the problem persisted with 10.15.3 so had obviously occurred beforehand (I don't reboot this computer very often so hadn't noticed).

Poking around in my Music folder I realised that recently macOS had changed where it was putting my files. Most of my music was in Music/iTunes/iTunes Music and within that, a fair bit was in Music/iTunes/iTunes Music/Music - for reasons known only to macOS. However, the items I had added in the last 2 weeks were instead in Music/Music/Media.

I wondered if the fact that the library was now spread over two or three locations was the key, so I unified it into one folder (Music/Music/Media) and added it to Music. This restored my music, obviously at the cost of much metadata, organisation, and all my playlists. However, when I rebooted the library was empty again.

I think the most significant detail is probably that my Music folder is not on Macintosh HD as it is too large, but is on an external SSD and aliased. I appreciate this is not ideal, but my iMac has a 1tb SSD and that is about the size of my Music library, so I have to keep it on another drive.

Does anyone know any steps I can take to get my Music library to persist beyond reboots? Recreating the whole thing every time I reboot is not really viable