the mobile-first approach is based on this process, in a nutshell, you should start designing the experience and the UI starting with the smallest device and scale the solution towards the other breakpoints.
In my experience, this process is not always frictionless, some UX/UI patterns are strictly related to the framework you use for delivering the native app (eg iOS and Android).
I believe that most of the effort will be allocated in designing an option for those native patterns.
it sounds like an interesting project, could you tell us more?
- which type of app is it?
- why did you choose to develop (and ship) the native version?
- why do u want to extend the experience to desktop devices?