Open Source for IoT Firmware development.
As I mentioned earlier that it is important to bridge the gap between and an IT professional and Embedded firmware engineer to turn ideas into reality in the world of IoT. Bridging the gap would certainly help to turn PoC into a real product.
While exploring the various Operating System I jumped upon FreeRTOS which is most popular Open Source platform for firmware development. Large community support makes it easier for beginners to adopt this platform easily.
Its fairly easy for an IT personal to adopt FreeRTOS. This is supported on many platforms such as arm, cortex,Atmel etc.
To start with I would suggest to go with ESP32 module and try out FreeRTOS on this device for developing IoT devices.