When working with heavily Maps based interfaces you might feel a little constrained by the base MapView Component that comes out of the React Native box.
At this point you might either start working on a full pack of new features, maybe over a declarative API, or just head to Leland Richardson's shiny Mapview component.

No pressure, take your time!

react-native-maps on GitHub

This MapView component is built so that features on the map (such as Markers, Polygons, etc.) are specified as children of the MapView itself. This provides an intuitive and react-like API for declaratively controlling features on the map.