Thanks for reporting. We're discussing this possibility on the egpu forums here: https://egpu.io/forums/mac-setup/sc...macs-on-macos-10-13-4/paged/22/#post-26327280
Feel free to contribute if you want, we might be able to see if a solution arises.
Ah, one more thing. Also on my iMac, if i completly remove the internal AMD GPU, the internal screen is not displaying anything anymore. So this screen is seems to be hardwired to the MXM socket.