Photos Don’t Upload from iPhone XR to MacBook Air

My MacBook Air is running the latest Mojave OS and my phone is at iOs13.5. My Photos library is stored on an external hard drive and I do not use iCloud Photo Library.

Until the past six months or so I synced whatever iPhone I had with my computer via USB cable. At some point I changed over to have the photos synced via WiFi, and it’s been working more or less fine since then.

Suddenly about three weeks ago the over-the-air syncing stopped. Neither the Mac photos album nor the Photo Stream have been updated on my computer since then. I could just fix this by connecting with a USB cable but recently that has been problematic as well. For some reason the Photos program doesn’t keep very good track of what has already been uploaded from the phone and what “new” photos are there to be uploaded. As a result I have wound up with many duplicates of pictures that were uploaded previously.

There are about 25,000 photos on my hard drive and there is probably 2TB of memory left on that drive, so space isn’t an issue. I don’t work with this stuff on a daily basis and it’s becoming increasingly frustrating to have some new issue crop up nearly every time I start Photos on the Mac. I don’t know of a way to force syncing without using iCloud and I may do that at some point but want to get the lower-tech issues resolved first.

So, two questions:

1. How can I move forward with syncing?, and
2. Is there some way - app settings, different photo app, etc. - to manage the photos I have on both devices without a lot of manual intervention? (By “manage” I mean have the Photos app on the Mac keep my library updated and free of duplicates.)