I just wanted to let anyone else struggling with MQTT (I can’t be the only one!..Right?) know that it’s pretty easy to set up on a Pi already running Hassio and configure that to work with Rhasspy (I’m running on a separate Pi in Docker). The instructions on Home Assistant (https://www.home-assistant.io/addons/mosquitto) were a little confusing for me but boil down to:
1-Hassio > Add-ons > install the mosquitto broker add-on and start it
2-Configuration > Users > make a new user in Home Assistant for the MQTT stuff
3-Configuration> Integrations > MQTT > the IP address of the Hassio system on your network, port is the default, and the username and password are the ones you just made in #2
4-Configuration> Server Controls> Check config > If it’s valid, restart the server
Then in Rhasspy settings, enable MQTT with the IP address, port, username and password you just set up. Save settings and restart. This all worked for me, hope it helps someone else!