Enhance Existing Kotlin APK Security

I have a production APK written in Kotlin that relies on Retrofit-based HTTP calls and Protocol Buffers for payload serialization. The core features are stable, but I now need precise, code-level adjustments focused on the client–server security layer and a few related Retrofit endpoints... (Budget: $10 - $30 CAD, Jobs: Android, Android Studio, Java, Kotlin, Mobile App Development, Security, Software Development, XMPP)