Interesting I had a look at doing MQTT in the client and that looks like a non starter.
Also if you search for retool realtime you will find that it's not supported as a concept.
So what ever you message store is DB/MQTT/REDIS your retool app will have to poll.
Now interestingly it seems retool are starting to support http/2 SSE server sent events for postgres. Using SSE fits very well with retool can only hope this is an ongoing and strategic choice to build responsive apps.