Thanks John and Zack for all the above. I was about post of having identical issues - intel Mac, eGPU with the highest card an intel model can run: rx6600 with 8GB VRAM. Clearly some issue with the current TRS22 build I am guessing - though it could also be Sequoia 15.3 update, as I installed...
Sharing this in case it is useful - I've tried lots of ways to obtain greyscale maps (use them at work as well as for Trainz content). Not many easy, free and good sources around, so when I discovered this I was pretty pleased.
https://tangrams.github.io/heightmapper/
Content and detail...