I have just done another successful sync without deleting any directories or reverting back to previous versions of anything. Instead of 20 minutes short of 7 hours, the successful sync took only 5 hours and 28 minutes for slightly more than 5000 photos. All the F** directories (and *.ithmb files within those directories) in iPod Photo Cache still have the same time and date signatures as the previously created files reported in my comment of 26/04/18 above. However, the Photo Database file has been updated with the time and date signature of the completion of the sync.
I have checked the iPhone photos and all appear to be in place under Albums with the same names as the sub-directories of my PC Pictures directory. I did not have any favorites or other markup so this is a simple test case without any other complications.
This second successful sync was carried out with iTunes 12.7.4.80 under Windows 10 and iPhone 6s under iOS 11.3.1 like the previously successful result on 26/04/18.
The only confusing aspect I found was that the progress bar for the "Reading Photos from..." step quickly reached 100% done and stayed there throughout the Reading Photos step. So it did appear that the the Reading Photos step had frozen. In my case it was not frozen. The step was simply incredibly slow.
Unless I have another failed sync, as others have reported in this thread, I will not try deleting the iPod Photo Cache directory.
Unless anyone has specific evidence that their "Reading Photos from..." step really is frozen (my Task Manager reported iTunes consuming 70% CPU throughout the "Reading Photos step), then others should try just running the sync overnight using 1000 photos per hour as a very rough guideline (clearly depends on processors and memory) for expectations.
Of course, this does not address the real problem that many have reported here where a very small Pictures directory simply fails to copy over to their phone or iPad. Nor does this case address the favorites issues. But this does somewhat illustrate the difficulties Apple is having in solving this issue. An intermittent bug really is the most difficult problem for software people. Until users can report sufficiently well defined cases that can be repeated every time, solving this issue will be very difficult.
Cheers
Ian