Lost 9K music files after Sync Library - Read Before You Do This

The problem has been resolved, but I wanted to share my experience here.

I have a large library of apple music which i have been adding to for almost 20 years. I listen on my phone as well, but mostly use my Mac, where I use it in an old-school ITunes way.

basically this If songs are missing from your music library after you turn on Sync Library - Apple Support happened, but nothing in the checklist on that page was the cause - or the fix

my library includes both apple music tracks and a lot of my own music uploaded from years (MP3, ALAC, AAC rips from CD). Note that its a legacy library folder from iTunes; not sure if that contributed to the issue.

For a month or so I had been getting the "playlist won't play across devices" notice when I would try to add new uploaded music to playlists. I finally checked the Cloud Status column (which I had been ignorant to; previously just looking at the cloud icon column). turned out that every Uploaded song from the last 40 days was "Waiting" status. Spent a few hours researching, most advice here and on the web was to turn Synch Library off and on. So I did so. After my library re-synched (which took quite a bit of time as i have 90K tracks), it was missing thousands of songs. I couldn't find any clear pattern in what was missing - oddly the majority seemed to be Apple Music tracks not Uploads, but there were some uploads missing.

This had me freaked out enough to call Support (having seen a post here where doing that seemed to resolve a similar problem), and while I was able to speak to a few people, none of them were frankly at the level of an Apple Music power user. Since I wasn't able to get any real insight, I suggested a Time Machine restore (I had one from about a week prior to the issue), and a support person helped me get started on that. Thankfully the Time Machine restore fixed the issue. But the moral of the story here is BEWARE the "uncheck and recheck Sync Library" solution.