After a rather large metadata edit, iTunes doesn't properly sync with iPhone (complicated by Library on Google Drive)

I have had my iTunes library of 140GB on Google Drive successfully for several years now.

Last year, I finally got an iPhone, and sync tended to work well for the most part. If an album showed on the iPhone and didn't play, I would just delete it from the library and sync again to get it back (on the phone).

However, the other day I found that for some reason iTunes/Music had put music into 3 different Google Drive folders. So I unticked "Keep Music Media folder organised" and reticked it hoping it would collect them all together. It didn't. So I manually moved files into their correct places (it was strange because it was mostly the first and last songs of random albums; and sometimes just a random number of songs per album).

While doing the above, I combed through almost 2,000 albums that I bought and have the mp3/m4a/etc files (not Apple Music) and decided to update a bunch of metadata, mostly Genre and Grouping.

Since then, iPhone sync fails miserably with errors like “Intro” could not be copied to the iPhone “Beery iPhone” because an unknown error occurred (-3234). After two of those, the sync quits. I can play all the songs that appear in the error dialog; they seem to change each time.

After fiddling with removing all the songs from the iPhone and sync again, e.g. untick Sync library, Apply; then retick and Apply, I gave in and decided to Restore the iPhone and start over from scratch. Some problems.

Any ideas? I am now attempting to make the entire Google Drive hierarchy Available Offline. And might just copy that to the default Music folder. While that's happening . . . I'm open to suggestions. Thank you.