Chat App with End to End Encryption

We have purchased chat App with the all the functionalities. Below is the URL from where we have purchased the App which is already developed in Flutter, we will need customization and maintenance support of the App another functionality which is not part of the App we will purchase is we need Backend CMS developed in PHP with MySql database to pull all the data daily on a specific time.
Please note that awarded company will upload and configure the App on Apple Store and Play Store.
Customization will be UI update. Awarded company will also porpose 3 options of new UI for the App provided in the link below.
We will release the App in phases:
Phase1 : Text chat end to end encrypted without video and audio call.
Phase2: addition of Audio & Video call functionality

https://codecanyon.net/item/mightychat-chat-app-with-firebase-backend-agoraio/31819316

App Features Required:
Sign In – (Google, Email)
Dark Mode
Search Users
Change Profile
User Status (online, last Seen, single tick, double tick)
Video Calls
Call Logs
Change Wallpaper
Clear All Messages
User Profile of Other Person
User Status Update
Change Font Size
Change Enter key to Send Key From Settings
User Read/Unread Messages.
Unread Messages Count in Chat List Screen
Delete Sent Message

Technologies To Use:
Flutter
Firebase Cloud Firestore
Firebase Cloud Storage
Firebase Analytics
Firebase Crashlytics
OneSignal
Agora SDK for Video Call

Backend CMS to pull all the messages on daily basis on specific time automatically or manually.
Password protected Backend CMS to manage all the messages downloaded to MySql database with the search, sorting and CRUD functionality. Backend CMS will also download all the images, audios, videos, documents shared during the conversations to keep the copy of the App's Data on MySql and alternative storage.