I've recently gone through the eGPU enclosure drama after reading an article linked here. I'm using one of the 2019 refreshes of a MacBook Pro, with a Radeon Pro Vega 20. I bought a Radeon RX 5700 XT at launch this week, paired with a Razer Core X, like you've got. (I can go into more detail, but AMD dGPU + eGPU is a ridiculous battle of knowing which obscure config setting to add to drivers to get them to install [Google for EnableFalcon=true]).

rEFInd successfully gets Windows to find the Intel UHD integrated graphics, so that's good. That let me uninstall the Vega 20's drivers and disable it entirely.

Even then, I still don't boot with the enclosure attached - it seems to ignore its presence? Unplugging and plugging it back in triggers Windows to go find all of the hardware, and then I have to wait 30 seconds while things grind to a halt as devices install. I'm eagerly awaiting AMD Navi 10 driver support in OS X, which I suspect will come around since people have found the names in kernel extension strings earlier this year.