But I doubt that my code is efficient :/ The battery (500mAh) is not even lasting a day when constantly checking for MQTT messages in loop function. I tried to implement a timeout for MQTT checks. But that results in messages being not received (see the comment in the loop function).

Maybe you could have a look at my code and tell me what I can improve?
https://www.codebin.cc/code/cm7n9k4ox0001jv03t45j3yaj:AHZXQjLPQ75nXtJ9nq4tuXzUQrWn6hrjDaYAsNySRNT5

Btw, I'm using UIFlow 2.0. Started using the UI but after looking at the autogenerated code, which was a total disaster, I started to write and optimize the code myself.