Do you actually have to pay for YouTube Music? I can use it perfectly well in the browser with uBlock Origin. And on the Phone there is Metrolist (yes those client have the possibility to stop working, but for me at least everything works mostly smoothly, and if google manages to block them there is still Morphee using the official client)

I guess you cannot upload so many MP3s in the free tier? But if you have so many I would follow other peoples advice of self-hosting Jellyfin/Navdrone if you are able to. Or depending on your listening style maybe actually just copying them to your phone.