> The best bet should be a core with the domain with a UI shell. And then you swap the shell according to the platform.

I've rarely seen that turn out very well. Typically it works ok on whatever desktop main developers use, and not so much on the others. That means using multiple frameworks, witht their own idioms and quirks and having to repeat a lot of work. Unless your UI is very simple it is pretty expensive to maintain multiple separate versions of it.