App MVP based on Cross-Platform mobile framework & low-maintenance backend service

We are looking to build a quick MVP for a mobile application and backend that digitizes the interaction between clients and their service vendor. The biggest challenges are handling a large number of picture uploads with storage, repeating step-by-step instructions configurable on a per-user basis and local reminders/push notifications based on user behaviour. Wireframes and designs are already being worked on. We want to start a pragmatic, lean development process and start user testing as soon as possible. We need app development as well as a fitting backend. The data we will handle is highly sensitive, we, therefore, want to utilize, secure and proven existing services for AAA as well as hosting as possible.