Is the cost estimation by the architect realistic?

Now one should compare what the general contractor offers or what is still missing.

The architect's breakdown already includes a lot, not just turnkey, but rather ready to move in including some additional construction costs. (33k floor coverings... are you sure that all fits like that?)
If the general contractor offers as usual turnkey and additional incidental costs are added, then the prices approach each other.

Note that the breakdown is not complete, I do not see any incidental costs for the earthworks, even if you plan without disposal.
You want to skip the remaining architect phases? Sure?

With the plan to award the trades yourself I would still start with the architect.

The general contractor has offered to the same extent. I know the list of craftsmen and have also pointed out deviations from his construction service description to him, so that he priced them directly. Additional construction costs are another €40k for him. In the cost calculation from the architect, there are just under €40k for earthworks in the shell construction. I am not yet sure exactly what I want to leave out with the architect. But I actually don’t need a tender from him if I award most of the trades to acquaintances or do the work myself. That would be wasted money. The same applies to construction management – having €30k or not having it. For example, I also have someone who does it on an hourly basis. I’m also wondering if I need detailed planning. According to his own statement, he is not really good at HVAC; usually, he brings in a plumber who gets the contract and then does the planning. I would like to have the electrical work planned by an electrician who is familiar with KNX, because a standard planning, where every electrician is tearing their hair out, is no use to me either. You meant that the costs for the floors are too high, correct?

Apparently, you are investing heavily in the structural engineering and the windows? That really adds up. Is the carpenter responsible for the staircase?

I spontaneously thought that over 100 sqm of usable space might be a bit exaggerated, even with the double garage included... that leaves 40 sqm in the attic and 59 for the remaining basement rooms. Question: does that include living space?

Extravagantly designed houses are usually not suitable for a general contractor - we have always been able to read about real problems with that here in the forum...

I would not exactly describe the architecture as statically complex. Also, with the windows, you could certainly make them bigger, wider, higher.

You could certainly go bigger, wider, higher with the windows as well.

..., but that would cost you over €60,000 extra.
And what about the usable cellar space? I have to be honest: I wouldn't build it with this price/performance ratio.
145sqm living space... and an immensely large cellar...

That's what this is about. This is an estimate from the architect, which I myself consider too high. And I would like to hear - hopefully similar - opinions on this. I don't find the basement that immensely large. About 13 sqm are possibly going to be an office, hence also the light well at the entrance front, a washroom with 8.7 sqm, a spacious storage room with 16.7 sqm, an 8 sqm hallway, and the double garage including a technical room of nearly 8 sqm.