All right guys -
I have been in a remote wilderness since I last posted here, so I took a while to get back to troubleshooting this, but wanted to tell everyone that I fixed this problem today, and it took very little effort!
I figured that the issue had to be with my Photos app on the laptop, and not with the phone, or with the new Finder sync. I have all of my photos (22,000) organized perfectly into folders of where each was taken. All of those folders are in Finder on my mac, and are in Albums in Photos. I sync everything from Photos to my phone (versus syncing directly from the folders in Finder). When I look at each of the 100 or so Albums that I have in Photos (under the Albums>My Albums menu on the left), there are NO DUPLICATES AT ALL in any of the albums. They are all exactly how I wanted them to be on my phone...BUT, when I click on Library>Photos on the left I see thousands of duplicate and triplicate photos. They only appear there, NOT in any of the Albums that I created and thought that I was syncing.
I got this app from the App Store today:
https://apps.apple.com/us/app/photos-duplicate-cleaner/id592704001?mt=12&ign-mpt=uo%3D4
And I carefully followed these instructions on how to use it:
https://www.idownloadblog.com/2016/02/15/how-to-remove-duplicate-photos-on-mac/
It found that I had over 22,000 duplicated and triplicated photos. They ONLY show up in the Library>Photos section of Photos though, which is what your phone is using when is syncs.
After deleting all of the duplicates, I synced my phone again and it went from having 47,000 photos on it, to having 22,000. No more duplicates.
Hope this works for the rest of you. This wasn't a macOS Catalina issue, nor was it an iOS issue. It was an issue with Photos having thousands of duplicates, seemingly from me not importing them correctly when I did it.
