Windows being Windows... tell me something new
I would remove the GT 120, its probably causing most of the issues (I know for fact that it does not work properly in EFI mode and will BSOD windows the moment the drivers kick in, any Tesla based Card will bring down windows in EFI mode)
also what happens if you download the latest 1803 ISO from MS and use that to install windows 10 BTW?
It seems you are exactly right. After I remove the GT120, I can install Windows with just the 1080Ti flawlessly.
In fact, I am very new to this GT120. I was an AMD guy, until half years ago, I decided change to Nvidia because I do more gaming and video encoding than video editing now.
And only less than a month ago, I got a used dual processor with the GT120. So, try to run GT120 + 1080Ti combo. It works well in macOS, but really not that good in Windows. And I really have no idea it can causing trouble in EFI mode.