Fitness App Revamp: React Native, Node.js Expert

I am looking for a freelancer / Individual (companies please avoid) with good experience in React Native and Node JS to revamp our STAMIN fitness application which is totally developed in frontend with minimal backend integration such as payment I am seeking a freelancer (individuals only, companies please refrain from applying) with strong experience in React Native and Node.js to revamp our fitness application. The app is currently developed primarily on the frontend with minimal backend integration, such as a payment gateway using Node.js.

The main challenges we face include performance issues and certain non-functional features. The freelancer will play a crucial role in restructuring the entire architecture, implementing development and deployment line, fixing bugs, and collaborating with a small team of young developers.
gateway using Node Js. The current challenge is the performance and some features are not working. The freelancer should act as a contributor and lead restructuring the entire architecture, fixing the bug and collaborate with small team of young developers.

Before proposing, please note that the application is extensive and already contains over 100,000 lines, and it is currently live. The STAMIN application is also in iOS, Android and Web. There are 4 profiles (Users, Gyms Clubs, Trainers and Super Admins) with many innovations & features