To clarify - my experience has been that if I start Mach3 before the control box is switched on, or don't wait for 10s or so for the IP/M to start up, then I have to shut down Mach3 and restart it. myCNC and the ET6 don't care which order they start in - they just connect.

Frustration with your system understood - I would prefer Windows to Linux myself, but I can run with either. It's only the fact that I have seen it running on Win10 on a couple of systems that lets me believe that it should work. In my case, it seems to run reasonably OK on Windows (I've run it for a fair while but mainly for testing/configuration and only a little bit of cutting) on two machines, but crashes almost immediately on the workshop PC and I never even see the main GUI.