Use AMD Pro Render instead of iray for pathtracing. Pro Render supports both Nvidia and AMD cards as well as great support for Mac using Metal2 backend.
Substance has support for NVidia gfx ards and also IRay. Substance should support OpenCL and Implements Pro Render so AMD gfx users can use their gpu compute power with Painter and Designer.
Better yet have a choice to switch between IRAY and Pro Render, Maya, Blender, 3DS Max etc... have multiple render engine support too.
this one or some other more hardware agnostic solution if its any available without paying leg and arm to implementing it...
I would not change iray for nvidia users. Since Pro Render is extremely slow on Nvidia. At least one for blender.