I can fix your project using Flutter SDK with Full-time and have implemented SignIn by Google,Facebook, Instagram with firebase.
Flutter Skills:Firebase,JSON and serialization(Backend),platform channels(platform integration),FlutterFire(third party platform sdk),StatelessWidget,StatefulWidget
I can build your app with high quality and you will be satisfied about my working result.
If I have chance of helping you, I will start immediately.
I have ability to help you using skill you required and can be realized it anytime.