I'm sorry but I don't run an NVIDIA card, so cannot really give you any advice. Have you tried removing the card and running the computer off the built in iGPU? For that you'll need to trash the nvidia drivers and shut down. Pull the card and give it a go.
I'm on 10.13.4 and all works fine - did it through App Store. You'll find with my guide most system updates just apply fine, and the only thing that will probably break is NVIDIA drivers and sound. Sound is easy to fix, and NVIDIA I presume just as simply, by using nvda-disable=1 (or whatever that switch is) at boot, until you can update the NVIDIA drivers.