Quick update - left the machine on (no sleep for it) overnight and the magic sprites did their thing, and a I woke up to almost a full compliment of songs, playlists and ratings. Huge sense of relief and phew!
Weirdly, would estimate that less than half a percent of tracks are still missing (I deduced from one of the shorter playlists by comparing same on the phone, there's a song by Alexandra Savior that I adore but has disappeared; the rest of the album is there, but not this one song) but overall pleased it's back. Have a theory that the 'waiting' cloud status thing is possibly relevant - in that it needs to match and / or upload those tracks before committing to adding everything else back to the library, hence why it took so long to resolve? Dunno, just guessing.
So, yes, just sleeping on it (and not letting the Mac sleep at all) worked here.
Cheers, David