Update on my situation. After doing a full phone reinstall, my first sync worked fine but took forever. The count between the Mac photo library and those on the phone were very close. Upon second sync, duplicates of random photos appeared but the total photo count on the phone actually decreased. A third sync actually introduced more duplicates and in some cases triplicates. In all cases, I am just syncing the library as all photos with no sub folders. My photo library is stored on an external drive for space reasons as I have over 35,000 photos.
When comparing duplicates and triplicates that have appeared on red on the phone, oddly there are cases where one photo contains date and location metadata but the other does not. I’ve also noticed that in some cases the duplicate image is slightly difference in balance (color, contrast, etc). I have not edited any of these photos or created any duplicates. This only appears on the phone.
Also interesting is that since catalina, any photos that I add to a shared album from my Mac will appear in the shared album on all devices, but if I add photos from my phone or iPad, or if someone else adds photos, then they do not appear on my Mac.
Lastly, I have wondered if there is something weird happening in this whole sync process because of shared albums. In one instance, I noted that after a sync between my Mac and phone, that several photos from shared albums that I do not own but am just an invited member to appeared locally on my phone. These happened to be photos on which I commented. Very odd.
Ugh. Broken broken broken. I would never have bought a large volume phone if I knew I wouldn’t be able to use it for photos.