Is it worth paying the extra cost for KFW55 or not?

If [Sole-Wasser-Wärmepumpe], I would take a look at the subsidies from BAFA.

The heat pumps can also use [Wärmekörbe], groundwater, or trench collectors as heat sources. However, both are quite expensive and will never recoup the investments.

Heat pumps can also use heat baskets, groundwater, or trench collectors as heat sources. However, both are quite expensive and will never recoup the investments.

I don't see it that way. We bought, for example, system collectors (a package for self-installation). It costs about €3000 including excavation work, etc. Additionally, the brine heat pump costs about €1000 more.

In return, you get a €4000 subsidy from BAFA and have a better annual performance factor and thus somewhat lower consumption than with an air-water heat pump.

It gets even cheaper if you build a DIY trench.
However, this involves personal effort.

Just read up on it in the Rosa forum.

That's true. If you get an excavator including a driver on an hourly basis, have some technical understanding, and like to do a lot yourself, the trench collector is ideally suited. We implemented it at a friend's place on a Saturday. But it also requires a lot of planning, which should not be overlooked. And a heating engineer who somehow goes along with it.

Gas condensing boiler is not an option for us now. No gas connection. Geothermal energy sounds promising. But our soil is very rocky. It could become a huge cost trap. So far, no one has mentioned a pellet heating system. Does it work with underfloor heating or is it too hot?

That already works. It can be done with a buffer and/or modulating. But it would be my second to last choice. The last would be oil. It also requires a lot of storage space and is high temperature as opposed to the low temperature of the underfloor heating.