The good news is -- macOS to iOS syncing seems to be working now for me after completely deleting my "People" album on macOS and it's been working for about a week now when re-syncing. I did a bunch of other things over the past few months as well though...
I was getting syncing errors on certain video files off old cameras like the Canon S200. I deleted what I didn't want and combined related small clips into single files and encoded them as MP4 files with iMovie and ffmpeg. That managed to solve the video syncing errors.
The latest thing that I did was go through my music collection that has been piling-up since 1997 and fixed all the album art and metadata. Finder wasn't syncing _most_ of the album art for some reason and flagging various errors. For some tracks, I had to delete the the files from Apple Music and then re-add them for the new album art to show up. More Catalina bugs! There are so many bugs with Catalina to iOS 13 syncing on the latest releases.
Oh, and I ran into yet another iOS 13.6.1 Apple Photos bug yesterday as well. If you delete an entire photo burst, you can't remove it from the "Recently Deleted" album manually no matter what you do. I even went into "General" -> "iPhone Storage" and cleared the recently deleted photos there as well. Nothing works. That bug has existed since iOS 13 was released. Still not fixed and iOS 14 is about to drop. What are they thinking? It's crazy the number of bugs that still exist in iOS 13 right now.
As I was typing this, a big ~20k photos sync completed, and it looks to be correct with no duplicates or blank photos. Not sure why it has to copy the entire library over when I only imported six photos from iOS to macOS though. My music imported correctly again on today's sync (via a lightning cable) as well. Not sure why Finder has to copy ~20k photos when I only imported six from iOS to macOS and resync'd though. 🙃
I spent over fifty hours (re)-curating my Photos and deleting regrettable and blurry photos and videos and another thirty hours curating my digital music. Everything is in much, much better shape now though. It was a good exercise to go konmari the heck out of my photos and music. I also replaced quite a few music tracks with remastered or higher-quality encoding. I basically replaced all of the album art manually because most of what was there was in 300x300 format (or worse!), and a lot of the music I have is custom-encoded stuff from my tape-trading-by-snail-mail days.
The bigger project I considered was just starting anew with Apple Music, Apple Photos, and iOS. I'm definitely starting with a new iOS 14 this year rather than restoring from a backup as I have been doing since 2008.
In fact, I did start with a new Catalina install on a new MacBook this year _without_ restoring from a Time Machine backup using the Migration Assistant, but that didn't help my syncing woes because since I just imported my legacy Photos and iTunes music libraries that had been accumulating since 2005 (and the files themselves have been accumulating since the mid-'90's!) manually from a Time Machine backup off my old MacBook.
I can't imagine a normal Apple user going through these extreme steps to make things work though. It's crazy that Catalina was released in this state and never fixed for an entire year. I've never seen Apple release anything like this before.