I'm back onto this project now. I created a completely new Unity project in version 2018.2.11f1. I started re-creating my shader in ASE and got as far as adding a couple of nodes before the whole thing froze up again. Specifically, I was using the contextual menu to add another node. I had to force-restart the Mac.
The new project was a standard 3D one using Metal. I'll try switching the project to OpenGL to see if that stops the freezing. Otherwise I'm completely stuck and can't work on this. Any help is much appreciated.