I use a 18 mini with sonnet BB , I get a white screen when I boot OS X , I just re-plug the tb3 cable and I'm fine , however with windows I have to boot with HDMI in mini and switch to gpu when it boots.You can run an eGPU, it just wasn't officially supported. I've used eGPU's since 10.11 personally, it wasn't as easy back then.
The wife's 2018 Mac Mini + Sonnet BB + Vega 56/64 is working.
2019 iMac + Razer Core + Radeon 7 working.
My office 2018 Mini + Sonnet BB + Vega64 WC working.
Also I only use Displayport cables on any eGPU monitor. I just don't care for HDMI cables.
kind of getting tired of this song and dance with apple not supporting or testing eGPU or their OS anymore for that matter.