I am having problems with an iPhone 8 and an iPad Air. I rebuilt my Photos library on my iMac last week but it had no effect. Yesterday I did a complete restore on the iPad and set it up as a new iPad using an iPhone, which (given enough time) restores apps and iTunes purchases but not anything that has to be transferred via iTunes on the iMac. I don't use Apple Photo Library (too many photos) and > 90% of my music came from CDs. I connect to the iMac using a USB cable. Today I started a new sync, trying to only sync photos, at 2:40 pm. I selected about 14,179 photos to be copied to the iPad. This is not the first post-restore sync that I have attempted. During one part of the process, the message appears "Copying x of 13,528 photos." But the sync eventually stops and a eject symbol replaces the "sync-in-process" symbol. So I ejected the iPad and checked the number of photos in the Photos app on the iPad. It said 832 photos were on the iPad. Before I tried another sync, I shut down the iPad and restarted, and quit iTunes on the iMac and reopened it.
Second sync: Started at 3:04 pm. "Copying x of 13,385 photos." Number of photos on the iPad afterwards: 974. (I had to give it a little time to adjust. When I first opened the Photos app, it still said 832, but that changed after several seconds.) I shut down the iPad and quit iTunes again.
Third sync: Started at 3:32 pm. "Copying x of 13243 photos." iPad photos afterwards: 1116.
Notice a pattern? Each time, a sync adds either 143 or (more often) 142 photos to the iPad. If this keeps up, it will only take me about 90-100 syncs to get all 14,179 photos on the iPad. At 25 minutes each, that's barely 2 days! Working constantly, of course, no breaks for sleep or anything.
The fourth sync I had trouble with. Right in the middle, I logged into this site, and the 2-factor identification ******** got activated. I think the iPad couldn't handle it and the sync as well. And the Photos app on the iPad wouldn't stay open (kept crashing), so I couldn't check on the number of photos in the app.