How to boot from iGPU and diable dGPU?

Hey!

So I'm in a bit of a pickle with a High Sierra 10.13.6 install. I have an Nvidia card but want to run my system off the iGPU to see how it performs so I can consider upgrading to Mojave. So far, I am unable to get any output from the iGPU unless the dGPU is completely disconnected or if I have primary display in BIOS set to onboard (in the latter care, the system boot loops).

I have tried this Guide to diable the dGPU (option 02) and it doesn't seem to be doing anything.

I have tried simply removing the dGPU and plugging into the modo hdmi out but the system bootloops.

I added nv_disable=1 to boot and that definitely "dumbed down" the dGPU but I still get no output from the iGPU.

What gives? How can I get my system to ignore the dGPU and let me use the iGPU?

modo - asrock killer sli z370
cpu - intel i7 8700k
gpu - nvidia geforce 1070 ti