Hey all, applied most of the solutions here to no avail on an older iMac i7 with High Sierra and iTunes 12.8. Then I went to the iPad in question and forced a sync from the Wifi iTunes-synchronisation menu which remarkably prompted the minor software update, so it seemed. This was not long after the iTunes reinstall, and I'm not 100% sure my action is what did the trick. Imagine a little delay and it could have been the update as well. iPad hooked up with usb, but earlier invisible in iTunes, both connected to wifi and the iMac also networked to the TimeCapsule router. While invisible in iTunes, it seemed invisible in Finder as well from what I remember but it was visible in the System overview, usb section, and in Foto's.
Good luck, Bas.