You need to disable the discrete GPU in macOS, either with an SSDT-DDGPU.aml or this boot argument
-wegnoegpu.
The Boot Argument should work with WhateverGreen.kext to disable the Nvidia card in macOS.
You need to set your Intel GPU to work with a display connected, under DeviceProperties in OC or Clover config.plist. The normal setup when a discrete GPU is present is to have the IGPU set to run headless, i.e. with no display connected. As you are using an incompatible dGPU you need to set the Intel GPU as the main GPU for macOS.
AAPL,ig-platform-id > 07009B3E is used when the Desktop iGPU is used to drive a display
You may need to change the SMBIOS to match this setup, as it may cause issues if you use a SMBIOS that expects a discrete GPU to be present. Possibly use the Mac mini 8.1 SMBIOS.
You will need both GPU's enabled in your bios.
When booting the system you need to have a cable connected to one of the IGPU's display connections for macOS and another cable connected to a port on the Nvidia card for the Windows OS. You will then need to switch between the two outputs on the display/monitor when you are booting macOS and Windows respectively.