I would also re-think Nest, I fitted Nest Gen 3, I was told it worked with MiHome Energenie TRV heads, well it sort of does, the app will change both the TRV and wall thermostat together, but using the dial will not change the TRV, and using a scheduled temperature change will not change the TRV, they have release temperature sensors in USA, but not in the UK, so it has all this cleaver algorithms, but can only control one room.
Since Bosch do not support opentherm, pointless paying out for an opentherm thermostat. Either use Bosch's own modulating thermostat, or use a on/off thermostat which links to the TRV heads.
The TRV is king, this is the device which controls the room temperature, and tells the boiler how much heat is required, so a simple head swap
and you have a system working far better than Nest, the one shown cost me £15 each in 2018, there was a cheaper non bluetooth model, if you want geofencing these
can be set up to geofence, more expensive, think I paid £72 a pair, the Drayton Wiser is claimed to work out with clever algorithms how long it takes to heat the room, so you set when you want it at a set temperature and it knows when to start heating.
The boiler is designed to extract latent heat from the flue gases, and to do this the return water temperature is very important, so the TRV controls the return water temperature and the boiler uses this to work out how much to modulate (turn down) the output.
There is just one problem using TRV's, they can't turn the boiler on, so also can't turn boiler off as would not be able to turn it on again. But in the main this can be done with simple time switch. And it will cause the boiler to fully turn down, then cycle off/on using a mark/space control, so really no need for a wall thermostat.