Whats the fastest video card thats installed in a eGPU case which is compatible with macOS Mojave?
wutVega 64, but stock has been dwindling. Hard to find them still being sold new without markups. RX 5700 XT is not exactly in stock everywhere either, and it's 10.15.1+.
Depending on your enclosure, keep an eye on this list:
https://www.sonnettech.com/support/downloads/manuals/Compatibility_Graphics_Cards.pdf