Yes you can use Eclipse Mosquitto but I think it is only via Docker (I had it on my father’s setup), otherwise you can install it via brew (this is my choice), and it works fine since yesterday now, no restart necessary.
If it's installed on the same system as Indigo (and the MQTT Broker plugin), then just disable the Broker plugin and restart Mosquitto. They're both going to use the same port.
You will need to edit the Mosquitto config file and make sure it's using the default port and to set a user/password.
bSrvr1911:~ RSS$ brew services start mosquitto Service `mosquitto` already started, use `brew services restart mosquitto` to restart. bSrvr1911:~ RSS$
and it is listed as a process in the Activity Monitor, but it is not in the "Application" folder. I'm afraid I have no idea how to configure it? Is the default port 1883?