Mac mini 2018 EGPU under bootcamp - Error 12
I've unfortunately have turned off my T2 security chip entirely already, with the option to boot to external media for a while now. @i0ntempest, I don't have a monitor plugged into my HDMI, just the Thunderbolt 3 connector to my Razer Core X, which plugs into my monitor via DisplayPort. I have tried every port and connection type on my Vega 64 card with no luck either. 50+ boots and no go with windows via eGPU at this point.
The most reliable way to use an AMD eGPU in Boot Camp is through either apple_set_os.efi or automate-eGPU EFI. If the eGPU starts acting up with different errors, it's best to run DDU and remove the Radeon graphics drivers. Boot back into Windows through the EFI and install latest Radeon drivers.
Not sure if this has anything to do with this issue, but have you tried resetting the SMC? Back when I was using an RTX 2080 with my MB Pro 13 2018, I would on occasion leave it plugged in while booting, which is similar to what the mac mini is doing. After one of the earlier Mojave updates, and presumably the bootrom update which came with it, it stopped being able to fully boot this way, unless I set SecureBoot to disabled and external usb boot to enabled. I prefer having as much security as possible so I eventually switched these settings back. At some point later I had to reset SMC for an unrelated issue, and after doing so, booting with the egpu attached in bootcamp started working again.