Hi Neil,
I ported my system across from openHAB to HASS some years ago after several months of parallel running with one MQTT broker. Both openHAB and HASS connected to the same instance of Mosquitto allowing automations and other config to be ported across one by one.
Similar to openHAB 2.0, MQTT devices can be defined manually or using HASS auto-discovery. The latter works is much easier for devices that support it (e.g. older Tasmota) as you configure the device once, not the automation platform.
The MQTT messages you posted suggest at least these devices are custom ESP-based modules, for which manual MQTT config might be easier in the short-term.
For an overview, here’s a walkthrough of HASS Mosquitto broker Add-In, MQTT integration, Tasmota Integration. If you wanted to add autodiscovery to a custom device, there’s some tips and examples in this thread.
The hardest part of moving across from openHAB is shifting from a “Java-like” mindset (tabs) to “Python-like” paradigm (spaces, JSON/YAML). Consistent YAML indentation is painful at first, but prototype using the visual automation editor first THEN tweak. Most of my automations ended up a fraction of the size of openHAB once I’d masterd the dev tools to live-protoype Jinja2 templating functions (e.g. {{ object }}).
In general, HASS documentation is function with few examples BUT search for terms in this forum to find (usually) working code.
If this helps, 
this post!