I would probably also opt for the general contractor with a "fixed price" in such cases. Otherwise, you end up paying an energy consultant a fortune for little.
We negotiated the additional costs from single-family house 55 to 40 with a surcharge of €20,000. So thicker masonry, different windows, better roof insulation. Additionally, the photovoltaic system and the energy consultant are included. The latter is already fixed at €5,325. This includes all calculations, as we already have everything available, and the blower door test is also included. We also already have the commitment from the KfW. The contract with the general contractor was signed today.