How to recover local music library after unsubscribing from Apple Music services

I'm pretty sure this has been asked million of times, but I can't find a proper procedure to recover from the current disaster.

Preamble: I used to have a Music library with thousands of manually selected and imported songs (mainly from an old collection of CDs.). They were all maintained in the local HDD. During the years I created several playlists and ranked most of these songs to my personal taste.

A couple of months ago I decided to use an offer to subscribe at a reduced price to Apple Music. After the offer ended I decided to unsubscribe (I did not use the service enough). All my original library on my iPhone remained available, and only the music accessed online became unavailable (as expected).

Today, I opened Music.app on my Mac (the iPhone library is synced with this Mac). It welcomed me with a warning saying "Your Apple Music subscription expired" (or similar) and, to my great surprise, after clicking ok all my library simply disappeared! Not only the music I was accessing through the Apple Music service, but ALL MY MUSIC, PLAYLISTS, METADATA.... I can't really understand this behavior....

Nota that all the music files are still there, but all the metadata is gone!

How can I recover from this disaster? I tried using an old backup. When I load it I see everything as in the past, but the dreaded message (subscription expired) immediately appears and wipes all my data! Is quite frustrating. Clearly, is a malfunctioning of the system, and there should be a way to recover the original library without loosing everything...but how???

Thanks in advance for your help!