Considering Apple makes FCPX and they are specifically recommending the Sapphire PULSE RX 580 (which is available as an 8GB card), I would use that if moving to Mojave. It is easily powered via dual mini 6-pin to single 8-pin.
The WX 7100 is a nice card, but does not look like Apple will be optimizing FCPX for it at the moment. It should work, but many of the substantial benefits will only be on the Windows/PC side. And it's $300 more than the Sapphire PULSE RX 580 8GB currently. Wait a few weeks until Mojave is out of beta and there should be more info.
Your NVIDIA GTX 980 may be more than fine for Mojave if the compatible NVIDIA Web Drivers are released. I'm currently using GTX 1080 FE on 10.13.6 with mostly Adobe products.