Thanks for keeping the thread up to date, but it's really strange now for the cMP. We have a card that has macOS driver support, but can't POST on cMP.
Anyone tried if co-installed the 6000 series card with another supported card (e.g. RX560) on the 5,1, or with another Mac EFI UGA card (e.g. 7950, or even GT120), or no monitor connected when boot, etc. would make any different?
Or even more extreme, use it via the Titan Ridge as eGPU. I know this is far from ideal, but depends on the situation, this may be useful. And at least we know if there is a way to make it work (on the cMP).