Thanks! You should get a broker set up first and play around on localhost: this makes it easier to get acquainted with what you can and can't do. Experiment with the QOS and 'retain' settings to make sure you don't lose messages.
I did a writeup at https://petergarner.net/notes.php?thisnote=20190811-Lightwei... which should give you some ideas. As regards clients I'd recommend the cross-platform MQTT Explorer https://mqtt-explorer.com/ and for iOS, I've settled on EasyMQTT which also provides some graphing options. I don't use Android but most of the clients are good (and free). Hope this helps!
This is just excellent. Thank you so much. I have a jumble of collectd, statsd into a TICK stack, hooked into PageDuty for alerts, but it’s just all so bulky and weby, whereas your approach is clean and bespoke, especial for monitoring of a custom system/platform. I will be starting on this by end of the day. I like it.
Is it easy to display the last message received on a collection of topics? It would make it easy to watch a custom summary of a large system.