Blender 2.8 doesn’t react on ‘Prefer External GPU’ setting in app info (Mojave) when external monitor is plugged to eGPU. The temporary solution is to fire up Blender 2.8 with closed Macbook’s upper case. When Blender is running Macbook’s case can be opened and eGPU works as a main GPU. It works for me with RX570 eGPU.
Blender fired up ‘normally’ - lagging due to partly use Intel Graphics:
Blender fired up with Macbook closed - working smooth with RX570 as main GPU: