I've joined simply to post here and confirm. I've lurked since Dec '19/January '20 when I was trying to set up the new Mac Mini to be an occasional Windows gaming machine. At the time I failed no matter what I tried (EFI Boot loader, switching TB3 ports, extracting pci.sys file from 1903, installing 1903 etc) and I gave up, convinced that the only issue it wasn't working was Windows support. Now that there's been a couple of Windows major releases, I decided to try it again.

Mac OS function always was flawless and without any pain.

I only have (and want to use) one monitor (I know it's weird to you kids today). Windows setup was always going to involve changing input sources from Mac Mini Integrated Intel 630 graphics on HDMI port, and eGPU DisplayPort. In the end, this worked fine. Really, the only trick at the end was to disable on-board video once everything was installed. Thanks for the idea @lukasz_lazarecki.

1. Installed Bootcamp on the main Mac Mini SSD drive, using the latest Windows 10 20H2 iso from Microsoft. This forces you to disconnect all other mounted external volumes, of which I have many.

2. With display connected to Mac Mini HDMI, keep eGPU turned off. Boot into Windows and proceed with Windows Setup. Display will remain connected to Mac Mini HDMI port until the end.

3. Download official AMD Radeon Drivers (I got the latest version "optimized for Cyberpunk")

4. Restart and wait at the Login screen. Power on eGPU. When eGPU is powered up (Windows might ding to notify detecting new hardware) login.

5. Start AMD Radeon driver installation...it will just do the first part and ask you to restart. When you restart, power off the eGPU again.

6. Wait at the Login screen. Power on eGPU. When eGPU is powered up (Windows might ding to notify detecting new hardware) login.

7. Complete AMD Radeon installation. At this point Device Manager will likely show eGPU present, but yellow sign and Error 12.

8. Right-click on Intel on-board video and disable (yes, while still connected to it, it won't turn you off).

9. Shut down Windows. Turn on eGPU, connect display to eGPU (change input on your display).

10. Start Windows. You will likely not get boot screen, and it will likely take "longer" than normal and go through a weird cycle where the eGPU signal goes off, then on again, but after a little bit you will get to your Windows login.

11. There's no step 11...it all worked properly from here, going back and forth between Mac OS and Windows without having to do anything with display input...just permanently connected to DisplayPort of the eGPU.

I've reconnected _all_ my other USB, TB3 devices since then, and there are no problems so far. My Bootcamp partition is small, so all I've been able to try out so far is The Witcher 2, but I have an external SSD primed to load with The Witcher 3 and Wasteland 3 when they go sale...I anticipate I'll run them just fine. The Witcher 2 runs very well, even on "Ultra" at 1920x1080. No, I don't have benchmarks, because I'm old.

What happens if you disable PCIe 16x instead of iGPU in device manager?

Is there any method to keep internal HDMI port ‘alive’ / connected without error 12?

@susurs, I'm not entirely certain what that would affect. I worry it would disable/change my ability to use my external USB-C/Thunderbolt drive which is what I use for my game storage as the built-in OS partition for bootcamp is small. Of course, I also worry of "messing up a good thing" by fiddling with it before I "finish" The Witcher 3 haha

@susurs, The PCIe x16 connection is for Mac mini configured with the 10GbE port. If your Mac mini doesn't have this Ethernet port, you can definitely disable it. In order to keep the iGPU activated (for HDMI port monitor output) you'd need to use an EFI boot loader. I'd recommend the modified apple_set_os v0.5 for Apple T2 Macs.

Have you tried 20.4.2 driver with the Superposition benchmark? 

I get different scores with older drivers (This is 5700 non XT version, just reported as XT for some reason):