Sounds like it would be Apple's job to develop their own platform.

I think SwiftUI etc al don't work on Linux and Windoes and Android, right? While HTML works?

Give them a break. Why doesn't the OP just help develop a highly efficient SwiftUI extension for Markdown if it's that important? Remember that there are many efficient MD libraries for Swift/ObjC which would in any case be the sensible approach here (and it also explains why he can't find a SwiftUI alternative to his liking).