Trust this Computer - endless loop

We are an Apple family with a Mac, 2xiPad and 5xiPhones of various types. For some time now (running into years I think) we have been unable to sync our photos and music from our Mac to our mobile devices because every time we connect the cable, an endless loop of “trust this computer begins”.

I have tried everything suggested on the internet for people with similar problems. Yes we are running up to date software, yes I have tried restarting, yes I have reset location settings. I was convinced this was a bug that would eventually be fixed in successive updates - but still the same problem persists.

I saw one post on this forum which suggested I restart the Mac in safe mode and create a test account. I have done this now, and can indeed connect the cable using the test account without getting the message. However, two new problems have arisen. Firstly the device is now not recognised in Finder at all, and secondly I do not have access to my photo library through this test account - just some odd folders - Recents, Imports etc.

Any further suggestions?