Hi. I have almost same PC like yours . Z370 / 8700k / RX580. So I lost 3 days to make different config and try many many download config and same problem.. green screen on safari video. and wen I test with VideoProc sometime give H264 - 4K - OK .. HEVC -4K ok .. some time only thinking more then 2min and nothing.. some time on HEVC show 1080P or UHD - OK.. but still green screen on safari video. SO I thing main problem is serial number of CPU. May be is little different between series of Intel HD630 of 8700K. But I'm not sure. Because I make more then 20 machine with this CPU and there in no one with this problem. Hacktools / patch save on config and done. but on this case any time nothing. I remember before wen I was on High sierra I use FakeGFX and use Kabylake 5912.. I thing should try this kind of config with. but I’m not try it yet. At least I put machine on Smbios iMacPro1,1 and work like charm.
best regard
P.S. SO My CPU are from 8week , because from first 13week of manifactory of 8700K can overclock more then 5.0 easy with low voltage. after 13week also can hit 5.0 but with more voltage. I thing right here is secret of main problem of this IGPU problem