Another noob question: see the attached image of my thermostat. Would it be possible to switch it for a smart one? What would I have to check? If it is possible, what would you recommend? If is don't want any 3rd party app or account?
The Junkers boiler at my previous place had an emergency mode where it was heating when the contacts were open. Even their simplest thermostats put a load on the wires depending on how much heat is required - so it doesn't go full blast when you are only slightly under the target temperature.
So, this all depends on your specific boiler. While it might work with a simple relay, it might be far from optimal in terms of energy efficiency and wear&tear of the boiler.