Or you could use languages that allow you to share code, so you have 6 thin native UI layers on top of a shared cross-platform core with all the business logic

Or you could use languages that allow you to share code, so you have 6 thin native UI layers on top of a shared cross-platform core with all the business logic and most interactions with the external world. You can do it today with C# and https://www.mvvmcross.com/ https://www.mvvmcross.com/