Calculate massive house correctly

Sorry for answering so late, but as long as it is still possible, I take a little break here and there from the topic of house construction.

After an appointment with a construction company, I now have a rough estimate of what our house in solid construction would cost (about 10-15% more than a prefabricated house). As a further step, I was advised to go to an architect and complete the house planning (provided I decide on solid construction). After that, I would go back to the construction company with the plans to check their feasibility and adjust them if necessary. The house would then be built turnkey; however, the architect would no longer be involved in the construction execution, and the construction company would act as the general contractor.

Next week, I have an appointment with another turnkey solid builder, but I do not think there will be a major price difference in the initial rough estimate. However, this general contractor has their own architect, which of course has advantages and disadvantages; let’s see what they have to say.

Maybe a few words about the topic of KFW.
Obviously, the prefabricated house builders tailor their houses exactly to the KFW subsidy to make the subsidies attractive to buyers.
Since financing interest rates nowadays are almost at the level of the KFW rates, in my opinion, at least for a solid construction, you have to consider whether to make use of the subsidy at all.
That does not mean that you should not build energy-efficiently, but you can save yourself the proof for the KFW and set your priorities regarding insulation/heating system however you want and not necessarily try to reach a certain KFW standard. How do you see it?

Regarding KFW, I see it the same way as you.
As for BU and external architect: try finding one who makes you a plan and doesn’t do construction supervision, etc. Have fun.
A proper BU should have its own planner, whether A. or Ing., who is authorized to submit to the building authority.
I don’t yet believe that the solid construction will be more expensive than the prefab house. K.

what our house in solid construction would cost approximately (about 10-15% more than a prefabricated house),

This magnitude does not sound to me like an actual "higher price," but rather like "more honesty in the 'construction-side' services." At most, I consider half of this price difference to be due to genuinely different costs and/or margin expectations with truly comparable equipment.

Regarding the BU and external architect: find someone who makes you a plan and no construction supervision, etc.

Yes, execution drawings and construction supervision must definitely come from one source. Otherwise, you end up like with an involuntary decoration of the bathroom with drywall casing of pipes and the like. From the interface "approved design planning" onwards, the general contractor often skips the execution planning—conversely, it is very helpful for execution planning to know the executing teams. So if the freelance architect does it, then awarding the contract—preferably also to a general contractor—belongs to him as well.