Also, comparing dGPU to eGPU is not a very fair comparison.
e.g. My 1080Ti work very flawless under MacOS indeed. Everything is supported, extremely fast. But I am running it internally in a Mac Pro, not as eGPU. For you, the same 1080ti running as eGPU can be completely useless in MacOS.
It's the same card, same OS, but internal / external make all the difference.
In my above post (#17), as you can see from the graph. Running the same GPU internally (41 FPS on a Mac Pro 5,1) can have 2x performance that the same GPU work as eGPU (only 19 FPS, even on a faster / newer Mac Pro 6,1).
So, if you comparing your internal Vega 64 to an eGPU RX580, and then conclude the Vega 64 has 2x performance of the RX580. This is not quite valid.