Good graphics card for running UE4 on Mac

For any Mac users out there, do you have any recommendations for a video card for running UE4 on a Mac? I recently received a mid-2012 12-core Mac Pro 5,1 running High Sierra that I wish to use to do personal game development projects. It currently has a Radeon RX 560 card, since I’d like to have Metal-supported graphics in case I need/want to update to macOS Mojave. I was just wondering if there are any much better options out there that won’t break the bank.

There’s some stuff here about what graphics cards are supported:

From what I understand, at this point only AMD graphics cards are supported.