So the flow temperature is always around 30 degrees.
If I were you, I would collect exact data over the next winter and plan the new heating system accordingly: - Optimize/increase flow rates - Install heat meters: note flow, supply/return, heat amounts - Heat quantity on the coldest day/several cold days in a row (heating load) - Maximum required flow temperature on the coldest day (I don’t quite believe the 30°, but that would be great!) This way you can plan the heating system and also know whether a heat pump can be operated usefully for you or not. You have the luxury that the old heating system still works and you can therefore plan and decide sensibly.
Is it actually possible to do without passive cooling? Or is the effect in summer so pleasant that the 900 euro surcharge (plus tax) would be worth it in our case?
You already live in the house ;-) If the house tends significantly to overheating, an air conditioning system is more sensible. If it’s okay now, it’s basically just playing around. Last year in the height of summer, we tested cooling a few times with the heat pump (air-to-water heat pump). That’s quite nice, but it doesn’t perform miracles. With wooden houses, I can imagine the effect is bigger. The cooling was especially pleasant after the heat wave, to get the heat out of the massive house a little faster. For geothermal heat, I find cooling especially interesting/sensible from a technical point of view.