What I know for sure is that it is not possible to run external monitors on iGPU (unless, and that is the last straw, iGPU is tied to USB-C port, which is not my case). So I am left with NVIDIA. For that to work there are basically 2 solutions:
To install Qubes while in BIOS is “discrete” mode is set and then to install either nouveau or proprietary driver in dom0 (and one of them installed gives only one resolution and refresh rate and I can’t remember which one), and then to run Qubes in “discrete” mode exclusively.
To install Qubes in “Optimus” mode, then to run dom0 and whatever else on internal screen, while installing:
a) sys-gui-gpu with dGPU only passed through to it, and set there “parallel” Qubes there.
b) to pass through dGPu to single HVMs to get external screen.
A bit more on my experience, but without desired outcome
Anyway, I hope @b1two won’t forget to comeback with his setup to share here.