IOT Middle-ware Architectural Suggestion & Guidance

I am Android Application Engineer and also Have Embedded Knowledge. I want to make ESP32 Based Products Operating with Android Apps. I want Suggestion & Guidance for Selecting right middle-ware architectural support.

Must have sound Knowledge of MQTT, AMQP, Google IOT Core, AWS Iot Core, Azsure , RabbitMQ, etc.
Also Need Guidance Developing Android App.

Please Suggest me Which is
- Reliable Technology
- Low Cost
- Secure
- Scalable
- If it is possible to use our server

Hourly Rate will be welcome.