Greyed out songs after subscription to Apple Music (payed subscription) - sync problem

Dear Apple Music Community,

the German Apple Support hotline leader asked me to consult Apple Community with my problem as there are some professional Apple developers in this chat, that can possibly help me, so here I am with my problem. The problem is, that I cannot play and also not recover my private mp3 collection after upgrading to Apple Music's paid subscription model. As you can see in the screenshot attached, most of the songs in my library are greyed out (only exceptions in this screenshot: Mastermind and Worst comes to worst, which I can use) and neither I nor the support knows how to fix this or how to recover my tracks.

To provide context I will give a description of the history how this problem appeared.

I upgraded to a paid Apple Music subsricpiton a while ago and am a long time iTunes user. At some point I had to upgrade from iTunes to Apple Music because Apple didn't support iTunes anymore. I have a lot of MP3-files (7000+) in my iTunes/Apple Music library which are from my private collection and the transfer/upgrade from iTunes to Apple Music worked without any problems, but then I decided to give the paid subscription of Apple Music a try.

Once I subscribed to the paid subscription, Apple Music started to sync my whole library of tracks (I guess it uploads them so some server or cloud, but I don't know how this exactly works, Apple Music just does it automatically). As this took a super long time (several days with this amount of files) I had to take my laptop to work at some point and just closed it and left my home, so the laptop didn't have WLAN for a while. I was expecting the sync to pause and resume as soon as the laptop has WLAN again. But for some reason this didn't happen and now I don't know where the greyed out files are currently (are they on Apples server or are they still somewhere on my laptops disk or neither of the two) and how I can recover the files, so they are fully useable for me again?

Can anyone help me with this?

I would also be fine with cancelling the paid subscription as long as this doesn't lead to file loss. All I want is to have my private collection back and preferably in the playlists I had created over many years in iTunes/Apple Music.

The songs are greyed out in all my devices (MacBook Air 2022, iPhone 13, iPad Air).

Thank you very much in advance for your help!