Syncing between Mac and iPhone stuck at "Reading Photos library"

MacBook Pro (Retina, 15-inch, Mid 2015) macOS v. 11.3.1

iPhone 12 Pro iOS v. 14.5.1

I still backup and sync my iPhone via USB to my MacBook Pro. After the most recent OS updates, that no longer works.

I open a Finder window and plug in my iPhone. I select my iPhone from the sidebar and watch it begin Syncing as "Automatically sync when iPhone is connected" is checked. This is normally a step-saver, but now it's become a nightmare.

The progress bar indicates it is backing up. Then it gets to "Syncing (Step 5 of 6) - Reading Photos library." And that's as far as it gets. Even leaving it running overnight. And nothing works - the Skip Photos button does nothing. Clicking the X next to my iPhone in the sidebar does nothing. Even quitting or force-quitting the backup and backups-helper processes in Activity Monitor does nothing. Only unplugging the iPhone from the USB has any effect.

I was able to click the Sync button. But that just started the process all over. When the Eject symbol appear next to my iPhone in the sidebar and I clicked on it, it asked if I was sure I wanted to eject it because it was currently being synced. I click Eject, but nothing happened.

I can uncheck "Automatically sync when iPhone is connected" and all of the Sync photos, Sync music, etc. options; but the sync process never completes so those options never stay unchecked. So, every time I plug my iPhone in via USB, the process starts all over again.

As a result, I have not been able to complete a sync between my Mac and my iPhone. So, my Music playlists and Photos library aren't getting updated. And I can't trust the backup to be of any use.

I have seen this issue has been reported for the last several years. But there has never been a viable solution posted that I can find.

Please help.