Danfoss Ally thermostats have a built-in PID controller that does this, and can be used with Home Assistant via zigbee2mqtt. In theory, you could use aggregate

Danfoss Ally thermostats have a built-in PID controller that does this, and can be used with Home Assistant via zigbee2mqtt. In theory, you could use aggregate heat demand from the Ally thermostats to control a modulating OpenTherm boiler.