Cost estimation KfW 40+, calculation of full floors

As a builder and house construction contractor, one should not hope but always check and calculate.

With what assumptions? With what certainty?
I am now talking about energy prices, the weather, the durability of technical equipment...

we had hoped to save money in the long term with the 40+ version on electricity, etc. With enough solar and battery, we wanted to be independent from the grid. That's why 40+

Completely unrealistic. You will not save a single cent with 40+, i.e. photovoltaics and storage, quite the opposite. You can find calculations for this in this and other specialized forums. You will not achieve autonomy even with storage, because no electricity comes from the roof during the dark season and the storage therefore just stands empty and unused in the corner. Even in summer, you would need a very large photovoltaic system and storage to really get close to 100%. The investment would then likely be over €30,000, depending on personal consumption; you will definitely not break even.

I am also quite convinced (and have calculated it for myself... based on parameters realistic for me) that anything better than the current energy saving ordinance does not bring a financial advantage. We only have KFW55 because we were already practically there anyway due to the desired central ventilation system.

The leap from KFW55 to even higher standards, such as KFW40+ or even Passive House, is never economical, almost regardless of what horror scenarios you imagine regarding energy price developments. It is commendable if you decide to do it and the money is worth it to you, but the large additional costs cannot be recovered. Just like heating and electricity costs in the double-digit range per month already at KFW55. It also has no real advantage for the quality of life in the house, as even the current energy saving ordinance already has a high level of insulation.

In my opinion, it is also not worthwhile to want to build arbitrarily future-proof. If a major renovation of your house is due in about 35-40 years, then any standard installed today will be so outdated.... in 35 years, the now top-modern house will also have a completely outdated standard, whether it was KFW40+ or KFW70 "back then" does not matter to anyone.

not arbitrarily future-proof

Future-proof. With regard to the system technology, I only see the brine drilling with a certain sustainability, as it can quite well outlive the owner. All other components have lifespans of 10 (storage!), 20 (photovoltaics, heat generator), 30 years (ETICS).

The manufacturers of electricity storage often guarantee a capacity of up to 10 years. What is supposed to be sustainable about that? In 10 years at best the frame on which the storage stands will have paid off.

Not wasting energy outside would be sustainable. If I wanted to satisfy my green conscience in this respect, I would invest in the masonry. So if a few thousand are left over, better take a thicker brick to cushion future price increases for energy. That at least has a lifespan of >20 years and thus a glimmer of a chance to amortize itself. With ETICS I would reconsider again, as they are only good for 30 years, as far as I know.

That sounds anything but good. Currently, we are planning Ytong exterior wall 48cm. Decentralized ventilation. Solar and air-water heat pump. Brine-water heat pump is not possible due to water protection area level 2. Then maybe reconsider planning to KfW 55 with gas heating.