When I upgraded my late 2013 MBP to Mojave it became clear that Apple and NVIDIA weren't working together anymore: no more NVIDIA drivers or CUDA support (only Apple's own drivers available). It wasn't an issue for the MBP, as Metal is fully supported with the NVIDIA GPU and Premiere and FCPX work great.

When I went to install one of my apps onto my iPhone, I needed to upgrade XCode on my 2010 MacPro to support the latest iOS, which meant upgrading to Mojave. I've moved most of my video editing to a high-end PC, so figured the risk of any issues was minimal. The upgrade to Mojave went perfectly smoothly with no issues. I removed the NVIDIA drivers, then opened a terminal and manually removed the CUDA drivers. However I found all graphics and GUI was very slow. I ran Premiere Pro and there was no Metal support. A bit of research showed the NVIDIA GTX 980ti wasn't supported by Apple with Metal- and no hardware acceleration even for the GUI.

Checking out http://www.macvidcards.com/ (where I've been buying / having modded) NVIDIA cards for MacOS for years, I found that the older GTX 770 was supported by Apple. Fortunately I had one on hand, since moved to a PC built for 6-channel video streaming.

After swapping the video cards, the MacPro once again has a fast GUI, and after running Premiere Pro (latest) and using Metal with this much less powerful GPU (and less memory- only 2GB of VRAM!), Premiere is running at least as fast as before with High Sierra. Haven't tested for dual monitor 60Hz 4K yet, but even if only 30Hz with only one Monitor in 4K, that's good enough for now.

The other option is to use Radeon cards, however I'm not aware of firmware mods to provide a boot screen etc. Here's one example solution (with no boot firmware support): https://eshop.macsales.com/item/OWC/MP1012R580V

For PP CC and FCPX, which don't use a lot of GPU RAM, the GTX 770 is probably fine. For Resolve or other apps which need a lot of GPU RAM, the Radeon is a better choice (or use a PC with top of the line hardware).

While there's pros and cons to Hackintoshes, recent advances in virtualization can remove (apparently) all the cons:

It's quite a bit of work to set up, however they appear to be saying that it won't break with Apple updates, as a typical Hackintosh may.