But install a controlled residential ventilation system?

What kind of wall structure did you have in the prefab house?

Just checked:

from outside to inside:
5mm diffusion-capable fine plaster
60mm thermal insulation board (Styrodur)
13mm wood-based panel
160mm solid timber stud framework with 160mm sound and thermal insulation (mineral wool)
0.2mm vapor diffusion retarder
13mm wood-based panel
9.5mm natural gypsum board (Rigips)

Total wall thickness 26cm.

Whether prefab or solid construction does not matter. A house properly built according to current standards is airtight, and air can only (AND MUST) be exchanged through controlled residential ventilation or intentional openings (often e.g. window rebate). Ventilating every few hours is rarely feasible, and as far as I remember, also not permissible on its own.

Yes, it is permissible - a controlled residential ventilation system is not required.

It is allowed to include the operator of the system in the ventilation concept. If it is clear that the operator can ensure moisture protection by ventilating independently, this is sufficient for the concept. However, he must be clearly informed that without his cooperation the concept cannot be implemented. No planner will realize such a concept, as disputes are inevitable in case of damage. I myself do not plan a system only with the operator or owner of the property. But nowadays it is hardly possible to ventilate intensively about 4 - 6 times. Therefore, one can hardly do without a ventilation system anymore.

We are also facing the decision of whether to install a controlled residential ventilation system or not. We achieve Kfw70 through solar thermal energy, which already costs more than €8,000 extra. A controlled residential ventilation system would then be an additional €12,000.

We keep reading that all rooms must be ventilated multiple times... Is it really that extreme? What about people who are employed or travel for several weeks? Not all houses without controlled residential ventilation get moldy, and hardly anyone would set an alarm to open a window, right?