you don't need to use High Sierra for eGPU support. I'm running Sierra and (after some helpful advice from @chrisbmx116) my laptop is happily driving a watercooled GTX1080Ti over TB2, housed in an Aikito Node Pro.
Edit: it's not small though. About the size of a mini PC :(
Plenty of cool stuff at NAB this year but figured @chrisbmx116 etc would be most interested in:
https://www.thesilo.ca/powerful-mac-gamer-external-graphics-solution/