Cross-Platform Hostel Management App

I want to run every-day hostel operations from my phone, so I need a clean, dependable mobile app built for both iOS and Android. The basic flow must cover room allocation, check-in/out, and real-time occupancy at a glance, with data syncing instantly between the two platforms.

A straightforward, well-structured codebase is important to me because I plan to keep adding features after launch. Feel free to suggest the stack you are most comfortable with—Flutter, React Native, or fully native—as long as the user experience remains identical on both systems and performance stays smooth even on older devices.

Please outline:
• The approach you would take to keep the database in sync for iOS and Android.
• Any third-party services you recommend for notifications or payment flow (if you think those fit naturally).
• The key milestones you see from first prototype to App Store / Google Play submission.

Hand-off should include source code, concise setup documentation, and a quick demo video showing the main user journey working on each platform.