Hi, not sure wether you fixed it, but in case: you shouldn't enable the iGPU, it's the multi-GPU setup that is creating an issue from my understanding. So basically try in your BIOS to disable both the iGPU and 1 card (I don't know if the SLI cable will be an issue or not) and see if you can install like that. Or disable both cards and go with the iGPU for the install (of course plug your display on the iGPU in this case). If your BIOS can do that, you should be fine. Otherwise you can go back to Rehabman post, there is a way to do it without BIOS intervention nor physical removal, but that's much more complicated than what I just indicated (requires specific injection technics).
For those of us with no integrated graphics, I wonder if an amd card would work?I just finished a clean install of High Sierra on my Gigabyte Z170-HD3P. I was able to successfully install it by removing my nVIDIA GTX 660 (as I was having the same issue of being stuck in the installation when my graphics card is connected), and using the built-in Intel graphics. I copied the kexts and my config.plist from my Sierra hard drive and transfer it to the EFI of my High Sierra (after installing Clover), and everything is working fine as my old Hackintosh Sierra setup. (=