So, regarding NVRAM and SecureBoot certificates. Windows does not hash the firmware, and there is no such thing at all. It cannot do it in the first place, as it does not have a SPI driver to access its memory. However, it does write large NVRAM variables containing secure boot database (db, dbx, dbt, dbr, PK, KEK). I believe MacPro5,1 has the same issue as Insyde, very likely it has borked garbage collection and thus simply cannot handle that. Recent OpenCore supports ProtectSecureBoot option, it will abort any write attempt to these variables and should fix NVRAM issues when booting Windows in UEFI mode.
OpenCore also received a very serious update to output subsystem. For MacPro5,1 you need to set Resolution to Max, TextRenderer to BuiltinGraphics, ProvideConsoleGop to YES, Scale to 100. Leave the rest as NO/empty. I will add the Radeon VII users to the PM soon (let me know if I did not). In case the issue with no output is still present, I will need you to update to the latest OC, set the configuration as prescribed, and include OC directory and OpenCore log.
@astonius86, regarding multiple operating systems on the same drive. Currently you can either put OpenCore's BOOTx64.efi to some other place and bless it (unsure how well it works). Or remove Windows' BOOTx64.efi and add BlessOverride with \EFI\Microsoft\Boot\bootmgfw.efi (you will also have to disable HideSelf). The latter is covered in the configuration manual "12.1 Windows support".
OpenCore also received a very serious update to output subsystem. For MacPro5,1 you need to set Resolution to Max, TextRenderer to BuiltinGraphics, ProvideConsoleGop to YES, Scale to 100. Leave the rest as NO/empty. I will add the Radeon VII users to the PM soon (let me know if I did not). In case the issue with no output is still present, I will need you to update to the latest OC, set the configuration as prescribed, and include OC directory and OpenCore log.
@astonius86, regarding multiple operating systems on the same drive. Currently you can either put OpenCore's BOOTx64.efi to some other place and bless it (unsure how well it works). Or remove Windows' BOOTx64.efi and add BlessOverride with \EFI\Microsoft\Boot\bootmgfw.efi (you will also have to disable HideSelf). The latter is covered in the configuration manual "12.1 Windows support".