i-3s Real time chat & messaging

We are a software house developing school support systems for the Ministry of Education of Malaysia. We have developed a mobile apps using Xamarin Forms and a Web based portal for our users. What we need now is a real time chat & messaging module that will be integrated into our mobile apps and web based portal. Users are able to initiate the chat by selecting their contacts which are already pre-registered in our systems. The parents or teachers contacts also will be determined by the students enrolment. Meaning that, if you are login as a parent role you are able to see your children class teachers as your contacts and etc. The chat module also must be able to cater for both private and group chat. Basically we need a ready solution that can be integrated to our apps without any recurring charges on the usage. We want something like CometChat but less features. You build the back-end for us and we integrate into our front-end design

Features that we need as below:
1. 1 to 1 or group text chat
2. Rich media attachments
3. Typing & read indicators
4. Online presence indicators
5. Message history

Other requirements:
1. Chat message can be stored in Firebase Realtime Database or MySQL.
2. Mobile SDK that can be integrated with our mobile app or Web API that can response to mobile app and web request.
3. Complete documentation for the integration