Just to let you know that with the recent significant improvements on the mobile app version and the offline mode you really nail it and imo GTDNext is now, by far, the best GTD app on the market. I would only would like to see a better log of the completed tasks like in Nirvana. The archive function is quite poor on this aspect, but not a showstopper though.
My concern is the future of the application. Are you convinced that your current pay base will be enough to continue the development and to mantain the service? It may be difficult to attract more users without a native android and iOS apps.
PS: the app and the website have in the bottom reference to 2017. This gives the impression that the app devolpment is more or less stalled, but I do not think that is the case...