He wrote me in an email:
“I had the 5700 XT installed along with the Vega II Duo when it caused a crash at wakeup.
I have not tried running with the Pro Vega II Duo removed and the 5700 XT as the only GPU — mainly because it has no EFI with Apple Startup and Boot screens. I only use it as an secondary GPU — and only for measuring its performance compared to other GPUs.”
Thanks a lot.
So the only solution is going with the 580 MPX and plug it into another display for boot screen and use the 5700 for the main working screen (or the other way round if you use a TB3 display and your software can be selected to a decent GPU that's not even hooked up to a monitor).
And hope that Apple finally provides drivers that work at least the few AMD GPUs on the market.