Why is Apple Music not syncing my library?

My library has over 9,000 songs but stopped syncing a few months ago except for about 300 songs. I contacted support and spent over an hour and a half with someone troubleshooting and trying various fixes with no luck. I never heard anything back and contacted them again recently and still no fix. Anyone else having this issue or know of a fix?

[Re-Titled by Moderator]