Yeah you have to set two things in the firm ware to boot other Ones on the 2018 MacMini.
1) You have to set it boot from external drives.
2) You have to set it to allow to boot other OS non-trusted by Apple (something other than Windows)
After those two you are set, however Batocera is being a B-Word to figure out why it doesn’t boot or it doesn’t boot right. I guess I will have to keep working on it, I was kinda hoping that someone else was having the same issues and where able to fix them.