Expensive plot + single-family house 155 sqm + cellar KFW40+, financeable?

What exactly are you missing in the additional costs? No extra items are listed in house construction. I can understand the objections regarding the kitchen and furniture. We would take it easy with the furniture at the beginning (I know, that is always easier said than done). For the outdoor facilities, our calculation is as follows: If we start building in 04/05 2021, we can move in during winter 21/22. Then in spring 2022, we would only do the lawn and possibly a hedge along the street. Carport to be built independently.

From when exactly do I have to start repaying the loan?

Interest is always due immediately from the first disbursement. The start of repayment can be freely agreed upon (well, somewhat freely – the bank does want you to repay at some point). Normally, repayment starts in the month following full disbursement.

Let's start 04/05 2021

At that time, I wouldn't give you a loan ;-)

Earthworks: 3000€ (sewer works are included.)
Photovoltaic system: 12,000€ (I know the managing director of a company in the next town through a friend, hoping it will be this price or even cheaper.)

The earthworks are too cheap compared to the standard, especially with a basement. We pay nearly 15k for the best sand. Soil survey costs at least 800€, including the determination of the Z-value for disposal. These topics are usually not included in the general contractor's offer because of the risks from the soil survey.

My 15k earthworks for the basement are already very cheap!

Regarding photovoltaic, please don't expect it to be cheaper. The solar technician's son coached my wife, and he couldn't offer us more than 1000€ net per kWp. Gross was not under 12k. I would rather add another 1000€ to the calculation.

What exactly are you missing in the additional costs?

Construction electricity, earthworks, surveys, construction site fence, disposal/refilling of the soil - if all this is included in the general contractor's construction performance description, perfect. I would be surprised.

hm...

Outdoor facilities: €15,000 (of which €7,500 for paving, €2,500 for terrace, €5,000 for garden)

That is too little budgeted for paving by a landscaping contractor.
I also find earthworks and this and that too little. But the gentleman may calculate differently than I understand ;)

Basement seems too cheap to me.

To me as well. I find it odd that apparently the basement is a utility basement, but equipped with venetian blinds and corner windows. If the basement is built within the thermal envelope, it will cost more.
Why even a basement? Is this a hillside property? With 800sqm I would rather build 10–15sqm larger to have 20sqm more actual living space.
I would also forgo KfW 40+. You have to calculate how much of the subsidy is left. You don’t really save much energy anymore with the Plus.
Listen to :)

Additional question: Filling, sanding, wallpapering, painting? Floors, such as parquet in living/bedrooms? If turnkey really only means placing furniture in the house, then everything is fine - but today that is a real exception.

The others have wonderfully added. In summary, I would assume that you probably won't come off cheaper if that was the hope. Rather, there are indications for more expensive construction if you stick to your program.