Once everything is up-to-date, turn off the computer.
Install Windows updates (many), drivers updates (optional, many) and reboot. Install Windows 10 via VirtualBox on the external drive. Prepare macOS to boot from the external drive. Hope you can make it work and maybe just maybe u find a solution for error code 12. It doesn't crash but I also still have code 12 (in device manager - go to your egpu), which means it's basically useless that it is working for now, since 'it doesn't have any ressource to work with (aka an external monitor. Install the AMD Drivers before plugging in your eGPU and disable the internal GPU in device-manager, note that I have a working windows 10 with my razer core x plugged in.