GPU co-rendering for OS X has not been implemented yet due to the need to re-code Cycles in Metal. There is an upcoming version of Cycles which should be ported to Vulkan, and as such it’s possible that the GPU acceleration will come to OS X as well through Molten VK.
In the meantime, keep a few things in mind:
CUDA/Optix accelleration gives you about a 30% speed increase, so you’re not quite halving render times; so don’t get the idea that if you could use Blender on Windows with a 2080ti your render times would fly. Render is still a time consuming task depending on your scene.
Do take advantage of Noise Reduction. Both NLM and Open Image Denoise are fully supported in OS X, and if used judiciously can yield great results with lower sample rates.
As suggested above, watch tutorials on how to optimize your Cycles settings, you can probably gain 25% or even more speed by being smart about your settings.
A 12-core Xeon is a very impressive 24-thread machine. CPU rendering is still king when it comes to complex scenes…just ask Pixar or WETA. Your machine should be able to tackled some pretty decent render speed results, so if you’re not feeling the speed refer to #3.