2020 27" iMac 5K (RP5500XT) [10th,8C,K] + RX 5700 XT @ 32Gbps-TB3 (Sonnet Breakaway 550) + macOS 11.2.3 & Win10 20H2  

System specs

• iMac (Retina 5K, 27-inch, 2020)
• 3.8 GHz 8-Core Intel Core i7
• 40 GB 2133 MHz DDR4
• AMD Radeon Pro 5500 XT 8 GB
• macOS Big Sur 11.2.3
• Apple bluetooth keyboard and mouse
• Steelseries Wireless Pro
• My Passport USB (for Windows backup)
• Wireless Gaming Controller CUH-ZCT2U connected via USB
• Envoy Pro FX connected via TB3 (for Time Machine) 
• Apple USB SuperDrive
• Windows 10 Pro Education 20H2 build 19042.844

eGPU hardware

• SAPPHIRE NITRO+  RX 5700 XT
• Sonnet Breakaway 550
• 2160p/4K UHD 55" TV

Hardware pictures

Installation steps

macOS 11.2.3

• Plug and play.

Windows 10 20H2

1. Disconnect all external devices (USB, TB3, etc.)
2. Optional : Connect USB mouse and keyboard (sometimes, bluetooth devices stop responding during install)
3. Install Windows 10 20H2 with Boot Camp Assistant
4. Optional: Delete D:\BootCamp\Drivers\AMD (we don't want to install the AMD video drivers, OSXRESERVED (D:) is used by Apple to install windows and drivers)
5. Install Boot Camp drivers
6. Click in the right side of the taskbar. Click the Boot Camp icon , then choose "Restart in macOS..."
7. With Clover Configurator : Mount EFI > Mount Partition > Open Partition
8. Open folder /EFI/Boot
9. Rename /EFI/Boot/bootx64.efi to /EFI/Boot/bootx64_original.efi
10. Download apple_set_os loader v0.5 and copy to /EFI/Boot/bootx64.efi
11. With Clover Configurator : unMount partition
12. In "System Preferences", set "Startup Disk" to BOOTCAMP and Restart
13. Press Command-R while restarting to enter "macOS recovery mode"
14. Set Security Boot to "No Security"
15. Restart
16. Install the latest Blue Enterprise Edition Driver from bootcampdrivers.com  (Red also works, but I prefer 5K on the iMac display)
    With "Factory Reset" (if you did not do step 4)
17. Power ON eGPU and connect TB3 cable
18. Restart
19. Enter Windows
20. Restart
21. Enter Windows
22. Disconnect and reconnect cable to eGPU
23. That's it... it just works

I did all the updates (Windows Updates + Optional updates + Apple Software Update) with the eGPU cable disconnected.

except the American Micro Devices, Inc. - Display

Reconnected all USB devices, except the TB3 drive.
Optional : disonnected USB mouse and keyboard (using apple bluetooth mouse and keyboard)

Then I reconnected the eGPU TB3 cable and did :

 macOS with eGPU to Windows 10 with eGPU

1. Disconnect TB3 drive (if you have one)
2. "System Preferences", "Startup Disk", select BOOTCAMP
3. Restart
4. Enter Windows
5. Restart
6. Enter Windows
7. Disconnect and reconnect TB3 cable to eGPU
8. That's it...  

Windows 10 with eGPU to macOS with eGPU

1. Click in the right side of the taskbar. Click the Boot Camp icon , then choose "Restart in macOS..."
2. Reconnect TB3 drive (if you have one)

Benchmarks

 Quiet Bios

Performance Bios

Internal Pro 5500 XT (Red drivers)

Comments

Reason for this build : I decided to replace my old 2010 iMac 21.5-inch i3 with a 2020 iMac 27-inch i7 + eGPU in January 2021, it was time...

Notes :
The apple_set_os.efi trick adds the iGPU Display adapter and changes the timing / order of the Windows 10 devices loaded during the boot process, even if iGPU is not really used (no display)
When just disconnecting / reconnecting the TB3 cable does not work, Powering OFF / ON the eGPU sometimes helps.