Support for MQTT QoS level 2 (each datapoint is sent exactly once using a 4 way handshake) would be nice as some of the networks I’ll be using will not exactly be reliable and this means better ...

The ESP8266, using the Tasmota firmware, sends a signal to an MQTT server running on the Raspberry Pi, which in turn translates it to a remote trigger on the Zigbee frequency with the dongle.