If I had infinite money I would use something like Xamarin.Native (aka Xamarin.iOS/Xamarin.Android aka Xamarin.Traditional aka Xamarin) to write a 90% shared codebase and develop my own UI dsl with the rest in native code. Codesharing for everything but the UI is a _huge_ maintenance win. The mechanism that Xamarin uses to do its cross platform means that you don't pay much of a performance cost to achieve this, because its mostly just a binding layer to native code (NOTE: not talking about Xamarin.Forms here).