Why even bother, this does not change the fact that macOS does not have drivers for the RTX GPUs. Besides that, does not make sense to run Windows only with a 12, 13 or 14 years old Xeon that will seriously bottleneck the GPU, build a slave/gaming PC or upgrade to a 2019 Mac Pro - where you can make the RTX GPUs work easily with Windows and have internal space to have two high end GPUs, differently from a MacPro5,1.
Btw, since the invasion on NVIDIA servers and 1TB leak last year, there are customising tools and OEM firmwares for testing/developing/certification of PCB designs floating around the net, the people doubling the GPU VRAM replacing memory chips are using these custom firmwares for some time already.
Anyway, the problem is the MacPro5,1 EFI 1.10 firmware that does not support HII and crashes at POST with any GPU that requires it without a fail-safe mode, like the NAVI 2x based AMD GPUs or the RTX 3xxx and 4xxx.
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