Hi @Boimb! Great to see you taking the initiative to create this topic 
The topics presented in the documentation are only incoming topics! We do not manage outgoing MQTT streams for now
There is a logic in the current API.
An example on the topic you mentioned:
After that, I agree with you, passing the external_id in the topic makes sense and is cleaner.
I like the implementation you propose! The idea of removing the json.encode/decode on the device side is smart.
I think you reversed it with the device. Gladys listens to what is intended for it, that is to say « gladys/master »