iBook synchronization.

iCloud Drive might be an option : Use iCloud Drive or iBooks to access your PDF files, ePub files, and books - Apple Support

I use a Mac with iTunes 12.6.4, so I just add them to iBooks on my Mac and sync them via iTunes to each that I want them on

How are you trying to sync it ? The Settings > iBooks > Sync Collections setting doesn't copy items between devices, it only means that if you have a book or PDF on two devices and you move it into a collection on one of them, it will be moved into the same collection on the other.