I'm planning to build a 32 relay IOT node next to my Raspberry Pi. The relays will be controlled by an Arduino Mega 2560. The Pi will run Home Assistant and Mosquitto so I would prefer to communicate with the arduino using MQTT. The Arduino and the Pi will be next to each other, the simplest connection would be over serial. I have found a library: https://github.com/vortex314/serial2mqtt it looks not easy to set up. Do you have any other ideas? Thank you!
Finally I have found the solution! Using mysensors (mysensors.org) I can use the Arduino Mega as a node and a serial gateway. So Home Assistant can control it directly.