I have a BAXI Luna 3 Comfort (Model 1.310 Fi) and looking at the same thing. Definitely no OpenTherm, but still decent change has Modbus, plus it has basic dry contact controls.
Spoke to plumber and they said that Hive will have a compatible controller/receiver, and once we have that connected, we have a chance to connect that to Home Assistant as described here:
You need hub, controller/receiver, and thermostat, but I can’t find where you buy the controller/receiver.
There are plenty others basic controllers with Tuya Wifi support that should work with just the “Dry Contact” control on boiler.