Rebuild Existing SMS Messaging App in Flutter for Android & iOS

Hello, I am looking for an experienced Flutter developer or development team to completely rebuild my existing SMS messaging application from scratch. IMPORTANT: I already have a live Android app published on Google Play Store. The current version was developed in Java and has accumulated many bugs over time. Rather than fixing the existing codebase, I want a completely fresh rebuild using a modern cross-platform framework (preferably Flutter) so that the application can be maintained more easily and published on both Android and iOS. My primary goal is to create a professional, stable, scalable SMS messaging application that feels as polished and intuitive as modern messaging apps. EXISTING APP REQUIREMENT: The new application must use the same Android package/application ID as my existing Play Store app so that I can release the new version as an update to my current users rather than publishing a separate application. Please let me know if you have experience handling migrations from existing Android apps while preserving Play Store continuity. PROJECT OVERVIEW: This is NOT an internet messaging application. This is a traditional SMS/MMS messaging application that sends and receives messages through the device's SIM card and cellular network. The application should function as a full-featured default SMS app. DESIGN & USER EXPERIENCE: I want the user experience to be heavily inspired by modern messaging applications such as WhatsApp, while maintaining original branding and avoiding direct copying of copyrighted assets. The app should feel familiar, clean, modern, and professional. Desired experience includes: • Modern conversation list screen • Chat bubbles similar to contemporary messaging apps • Smooth animations and transitions • Contact avatars • Search functionality • Archive functionality • Pin conversations • Message reactions (if feasible) • Swipe gestures • Clean settings screen • Dark mode • Light mode • Responsive layouts • Excellent performance on low-end devices FEATURES REQUIRED: 1. DEFAULT SMS APP FUNCTIONALITY • Set app as default SMS application • Send SMS • Receive SMS • MMS support (if possible) • Delivery reports • Read receipts where supported • Draft messages • Scheduled messages (optional) • Contact integration 2. PRIVATE / LOCKED CHAT SYSTEM This is the most important feature of the application. Users must be able to: • Lock individual conversations • Protect chats with PIN • Protect chats with password • Protect chats with biometric authentication (fingerprint/Face ID where available) • Hide locked conversations from the main inbox • Reveal hidden chats only after authentication • Lock specific contacts • Securely store protected messages The privacy system should feel polished and seamless. 3. CALLING FEATURES Since this is an SMS application, I do not need internet calling. However, I want integration with the device's native calling functionality: • Call button inside conversations • Call button on contact profile • Quick access to phone calls • Call logs integration if feasible 4. CONVERSATION FEATURES • One-to-one conversations • Group SMS support if possible • Message forwarding • Copy message • Delete message • Delete conversation • Star/favorite messages • Search within conversation • Media attachment support where possible • Share content into the app 5. CONTACT MANAGEMENT • Access phone contacts • Create new contact • Edit contact • Block contacts • Favorite contacts • Contact search 6. SECURITY • Local data encryption • App lock • Biometric authentication • Secure storage • Backup and restore options 7. PERFORMANCE The app should: • Start quickly • Handle large message databases efficiently • Work smoothly on low-end Android devices • Follow modern Flutter architecture and best practices • Be easy to maintain and expand in the future ANDROID REQUIREMENTS • Compatible with latest Android versions • Google Play Store compliant • Proper runtime permissions • Compliance with SMS and Call Log policies • Production-ready release build iOS REQUIREMENTS I understand iOS has restrictions regarding SMS applications and default SMS functionality. I would like the project structured so that the application can also be published on the Apple App Store while respecting Apple's policies and limitations. Please explain how you would handle the iOS version and what functionality would be available there. DEVELOPMENT REQUIREMENTS Please provide: • Similar projects you have developed • Experience with SMS applications • Experience with Flutter • Experience publishing to both Play Store and App Store • Estimated timeline • Detailed cost estimate • Post-launch support options DELIVERABLES • Complete Flutter source code • Android project • iOS project • Documentation • Build instructions • Play Store ready build • App Store ready build • Ownership transferred to me upon completion Please review the requirements carefully and let me know your proposed approach, recommended architecture, timeline, and budget. Thank you.