...
How I did this:
I looked at my notes when I was playing around with z390 in May and my best result there with i9 9900k of 2:47 seconds I got after I was able to get same view above with both card showing in Graphics/Display section.
Thing is that I read in my notes but forgot in the start that you have to
select IGPU as a primary display and not GPU and also to patch iGPU FB. What I did here I just pasted FB patch from my old z390 and it worked.
After that I was getting rendering of 5 minutes and 9 seconds which was 2 minutes faster but still not good enough.
Then I was playing around with different versions of radeonboost.kext also tried different version of WEG and then I said lets try without WEG and boom everything working as it should, rendering finally down to 2:38 seconds and fans kicking in while rendering.
...
@CaseySJ Imovie also not crashing and working just fine.
thanks and good luck