I've got a working iOS 12 Simulator now, thanks to Chris' tip.
First step is to find iOS.simruntime in the /Developer/Platforms/iPhoneOS.platform/Developer/Library/CoreSimulator/Profiles/Runtimes/ of Xcode 10.4.
(This is why you keep old ones around, FYI.)
I got it working but had to manually copy the iOS 12.4 simulator runtime out of the Xcode 10 app bundle.