Why not just buy a 2014 laptop? At one point during this past summer, you seemed prepared to walk away from macOS, what changed your mind? I'm not knocking you, because I chose the 2018 MBP to return back to the apple ecosystem.
I think a better solution is to run macOS in a VM, then you won't run into problems trying to run macOS on a GPU that is not designed to run.
I'm sure it can be done, but with apple only support AMD, I'm not sure its worth the effort. What will you be doing in macOS that requires a Nvidia card that you cannot do in windows?