Well that was a productive few days hacking on my #homeassistant setup...
* Got full integration with my #hestiaPI thermostat (via a REST sensor and REST switch) which eliminates my old MQTT approach.
* Got an automation script in place to handle the awkward Hestia boosting
* Got a fake switch in place to allow me to do repeated boosts while the fake switch is on.
* Set up a generic_thermostat object which reads from a #sonoff and sets the fake switch accordingly.
Lovely! Multi room next :)