Why does my iOS device get stuck during sync at "Reading Photos Library"?

After upgrading to Big Sur 11.4, all of my iOS devices would get stuck at the Reading Photos Library step and consequently, they would not complete a sync of either photos or (since it happens after) music. I spent a lot of frustrating time trying to fix it. Finally found this one response from a few months ago, where the user had the same problem and a solution that worked—removing referenced files from the Photos library.

https://discussions.apple.com/thread/252327179?answerId=254814519022#254814519022

That thread is closed, so I couldn't add an additional note, which I'll do here: Once you've created your smart album in Photos for your referenced images, you may want to do a screenshot of each of those images before deleting them so that (in the event the original image file is lost or never found again) you have a version of the image that you can then import back into Photos.

Removing all referenced images allowed my devices to sync properly again!