Gas or heat pump in a KfW55 house

Yes; only it has been at least difficult for us to find someone who wanted to do it. Take a look in the punkten forum, there is a Grabencommunity.

The annual performance factor of well-adjusted and dimensioned brine-water heat pumps should be at least 4.5, even for ground collectors. For this, the entire system including the source must be properly dimensioned and planned.

We realized our trench collector as a DIY project for under TEUR 3.

An HLB as well as a lot of reading in the pink forum is essential! A good HB etc. is also needed. It is certainly not the easiest way.

Warum wird abgeraten eine Wärmepumpe mit Solarthermie zu betreiben ??

ST requires large storage tanks to store all the heated water until it is needed, etc.
Heat pumps require small storage tanks to waste as little energy as possible for hot water production. In winter, the ST either does not run or runs poorly and the heat pump has to heat the large storage.

Better to use photovoltaics; you can do more with it...

That makes sense. For longer hot water demand, e.g. showering (10-15min), with a brine-to-water heat pump, heating is done only directly electrically (heating rod) or ??

We have a compact unit with 180 liters of hot water - heated to 45°C. So 20 minutes of showering is no problem... filling a bathtub and then showering is also no problem - the heating element is disabled. However, it depends on the showerhead - pure calculation: 9 L / min at 40°C would mean 22 minutes for us - unclear if water is already replenished during that time, probably not much. If you have a Rainforest Shower Deluxe shower with 22 L, then it's a maximum of about 10 minutes accordingly. But honestly, there's nothing against buying a 300 L hot water tank as well; the heat loss of the tanks is not that high - you just have to know if you need that.

..and you have a deep drilling?

I am considering the Junkers Supraeco T STM 120-2 (3-12kW) and 190 liters of hot water.

I used to think that a heat pump could never manage to reheate hot water reasonably without an electric heating element... with our gas boiler, you can already tell, and there the hot water is prepared with flow temperatures well above 65°C... (unfortunately, it does not display it).
In the parental home, I believe it was nearly 100°C.

With a brine heat pump, you are limited to a maximum of 50°C. Why do you want to burn your fingers so badly? It's just a waste of energy and unhealthy.

For making tea, we have a kettle...