"So potentially I could boot from and work in High Sierra on a MacMini if I am using an external boot drive? Is this the best known effective workaround..?"
You CANNOT boot a 2018 Mini using High Sierra.
No way, no how.
Impossible.
End of story.
What you COULD do is run a virtual machine (VMWare Fusion, Parallels, Virtual Box) and run High Sierra within it. That is the ONLY possibility open to you.