Replying to pharmercist (and others who have made progress):
I have followed this issue(s) since the beginning and just repeating there is no specific pattern or fix that applies to everyone. It is some form of serious corruption in Catalina, and/or Photos library when Catalina was installed.
Different unpleasant side effects for different people and different fixes work for different people, and no fix will work for many.
I couldn't sync to my iPhone 6 (IOS 12) reliably initially no matter what I did. Then there was an early update to Catalina and suddenly, after several repairs to my Photos library, it all synced. Folders and albums on my iMac sync correctly to my iPhone 6. And this has stayed reliable. My initial corruption is gone?????
Unfortunately the initial problem is messy & varied, and I don't think Apple can solve all forms of it. They need to provide a tool that sucks the photos out of Photos correctly and then imports them back into a brand new Photos library without any corruption. If you you do this sort of thing yourself, from backups etc. it works for some but not others.
If you are up to date with upgrades and have tried most of the suggestions in this forum and it hasn't helped, then nothing, except (unlikely) special help from Apple, is going to solve your problem.
But keep protesting and providing feedback in Apple Support. https://getsupport.apple.com