How does one correctly balance between the Building Energy Act and KfW 55, 40, and 40 Plus?

We plan to build with a double-shell construction and want a controlled residential ventilation system for comfort reasons. We are now planning for 55EE; the thicker insulation and the additional ancillary construction costs cause approximately 10,000 euros in extra costs compared to the Energy Saving Ordinance with controlled residential ventilation, i.e., for us the subsidy option is financially attractive and the incentive to do more than the Energy Saving Ordinance. If I calculate from the technical minimum for the Energy Saving Ordinance (without controlled residential ventilation) to 55EE, it is roughly cost-neutral. However, it also naturally depends on the wall construction.

How does one correctly weigh between the Building Energy Act and KfW 55, 40, and 40 Plus?

You can't really do it "correctly," insofar as there is a lack of the necessary objective philosopher's stone. Apart from the blurriness between established facts and honorable hopes, this topic can never be viewed completely free of taking a personal stance toward one ideology or another, which would require the allergy warning "may contain traces of subjective opinion" to be affixed. Therefore, be cautious of any claims that the recommended path is purely "right."
My personal opinion seems hardly worth repeating here: the forum search could certainly work better, but my member name in connection with "WDVS" or also "Controlled Residential Ventilation" still won't leave the seeker in the dark.
I can't contribute anything "objective" in that regard ;-)
But "statistically," I can roughly summarize from four years of roaming in this forum that some of my views are relatively widely shared here, namely: that the so-called "fat levels" "KFW," "KfW turbo," and "KfW double turbo," which are regarded as more eco-ethically noble than simply fulfilling the Energy Saving Ordinance, are something that must always also be worth your own money: these are all programs with subsidies that, following their intellectual fathers and mothers, are supposed to provide incentives or to encourage rethinking — but are never suitable (nor intended) to make you "rich" by using these programs. So beware of the false hope that the subsidy for the next stricter KfW "fat level" itself (or its combination with a granny flat or the like) will fully cover the additional expenses required or even throw a little bonus on top.
Conclusion, from my point of view: do what you think is right — for all I care, also build a plus-energy house including heat recovery even from the cold sweat of fear about whether the efforts might still not be enough to achieve all Greta goals — but don't dream. Instead, keep in mind Tim Bendzko's hint that before saving the world, one still has to check "briefly" 148,713 emails :)

With a thermal composite system, KfW55 can be achieved with thinner walls, which on our narrow plot would allow about 10 sqm more living space.

Oh no, not really. We are talking about thirty-six point five centimeters monolithic versus thirty-three point five centimeters as the sum of 17.5 cm brick and 16 cm insulation panels. You will never ever gain 10 sqm in a single-family house that way.

So if I have done anything right in our project, and I mean really right, it was the decision to involve a proper energy consultant (in our case Ing.-Büro/Sachverständiger) early on... especially since 60% of the costs are subsidized. In the end, we achieved KfW40+ with about 10K additional costs and received 15K in subsidies (however with timber frame construction).

Consumption is about 60% dependent on the user, 30% on correct heating planning, and 10% on the energy standard.

Well, I am a prime example that this actually isn’t true.

All rooms are permanently set to 23C and the bathroom to 25C, no night setback.
Ventilation happens whenever it happens, the heating is not turned down or otherwise adjusted beforehand. It has been running for years without special supervision or operation.

Therefore, I would consider myself an absolute worst case user.

The annual costs for the 112 sqm house with two people.
336€ gas
427€ electricity

No big system on the roof, just two collectors for hot water when the weather is nice.

I think that is absolutely okay.

Regarding the question of which energy efficiency level you should build with:
If you are young, 25 -> as efficient as possible

If you are around 40 -> it’s not really worth it anymore, better to invest little and be done with loans before retirement.

If you’re even older… 50 -> forget any additional insulation.

Expensive thermal insulation only pays off over time, I built young and therefore invested in KFW55.
If energy were three times more expensive by retirement, I can live with that. Without KFW55, three times more expensive would quickly turn into six times as much as today. Unbearable.

So I am a prime example that this actually isn’t true.

All rooms permanently set to 23C and the bathroom to 25C, no night setback.
Ventilation is done whenever it is done, the heating is not turned down beforehand or otherwise considered. It has been running for years without special supervision/operation.

Therefore, I would consider myself an absolute worst case user.

Why are you a prime example?
You pay exactly your price for gas because you use your house the way you do. Different ventilation and rooms at 20 degrees and you might be at 25% less gas consumption.
You determine your consumption with your settings. Or do you have a comparison with different usage behavior in your house?

Why are you a prime example? You pay exactly your price for gas because you use your house the way you do. Ventilate differently and keep rooms at 20 degrees and you might reduce gas consumption by 25%. YOU determine your consumption with your specifications. Or do you have a comparison in your house with different usage behavior?

Exactly, but 25% is not 60%.

I am also sure that the fact that I built with KFW55 has reduced my consumption by more than 10% with the same usage.

No matter how wrongly I use the house/heating, thanks to KFW55 insulation I pay quite little. And that is ultimately what matters.

Your post suggests that costs can be reduced by 60% through different usage, which is not true... well, unless you turn off the heating in some rooms and keep the rest at 20 degrees or less. Rather unrealistic. Just as you won’t drive a BMW M3 at 7 liters, even though hypermilers easily manage that.

All in all, I can only advise: everyone has to decide for themselves whether the costs for insulation can be recouped.
For example, if you are 50 and want to leave your child a nice house and have the necessary money, go ahead and insulate properly.
Otherwise, the place will simply be outdated and operationally unattractive after 30 years. You can’t expect much rent in 2050 if heating costs for a small property exceed 1500€ annually.