Construction cost per sqm, affordable vs. expensive providers, construction cost reduction

Because he believes he can save money with the funding.

- are there realistic options that I haven't considered and with which I can reach 2700-2800EUR (KfW40) or 2600EUR (KfW55)?

How many square meters of living space are we talking about?
1. In most cases, you can do without a planned basement, excessive additional storage space, rooms with only one use multiplied several times, etc., and plan more multifunctionally. A planned attic in a pitched roof can be converted later, kids can live well in 14 sqm instead of 18, good room planning with a well-placed staircase can save 20-50 sqm (see the now most boring standard floor plan currently hyped: staircase with mega hallway on the upper floor, but dissatisfaction with the room sizes on the upper floor (and then no attic, because townhouse))
2. Why does it have to have the KfW stamp? Please definitely calculate whether you can come out cheaper without it.
Psychologically, however, I say straight away: any subsidy that you don’t have to work for is spent multiple times – you also tend to invest in unnecessary house features again.

It's limitedly possible, I am not a professional – painting yes, flooring yes, drywall hmm yeah if that works, ...

You can learn a lot yourself.
We didn’t have cooking, having children, or hobbies in school either. However, I wouldn’t overdo it here, because DIY takes time and can limit your health.

However, you can also install floors in ancillary rooms later after moving in, if kids aren’t there yet, and do these rooms completely yourself.
And then there’s of course your own reflection: just because you don’t install the “expensive taste” doesn’t mean you have bad taste.
Instead of a mega 4 meter sliding door, two double patio doors are sufficient, a rain shower is annoying in the long run, brown wooden doors are not timeless!, standard stairs can also look stylish, a cheap pitched roof house can look “more expensive” or higher-quality with nice window structures than the standard villa with the world’s most boring window arrangement.
There are alternatives for spotlights.
Not everything possible in electrical installations has to be done.

And if you deal with taste, hype and hacks, then even with the mentioned savings measures it will be a nice, great house that offers the family a home.

If you then go for the bonus of the attic conversion after 10 years, you will have more quality of life than others whose basement is already cluttered ;)

- we are planning a single-family house, about 150m2 floor area
- equipment not the cheapest, but reduced to the essentials for cost reasons

The more upscale KfW efficiency levels would not belong to the essentials. Funding schemes are not designed to effectively make building affordable for everyone or to ease the price. Most of the money spent on building can be saved through smart planning, especially with terrain modeling – long before a comparison of construction service descriptions can come into play. Secondly, through discipline in house size and building design (bay windows, projections and recesses, beams, corner windows, and the like cost extra). Thirdly, by selecting the right construction company. Whoever wants economy should also choose economy (“turnkey” applies for example to Town & Country). Outfitting a low-cost builder to economy will be more expensive, and you also pay extra when downgrading, if you remove service and delivery packages from an upscale provider. This also applies to trades that you think you can get cheaper from a schoolmate. The “partial leather” concept also works for clinker brickwork. Unfortunately, we don’t know your building idea and your plot, otherwise we could be more specific.

However, floors of ancillary rooms can also be installed later after moving in, if kids are not yet there, you can do those rooms entirely yourself.

That is, for example, a good approach.

We also did not learn cooking, having children, and hobbies at school. However, I would not overdo it there, because EL is time-consuming and can restrict health.

Having cooking done by others costs money, and outsourcing having children is not everyone’s thing *SCNR*

Good grief... ypg... good thing I've been ignoring you for a while... what you "offer" on topics you have no idea about... especially when it comes to children... that's not even remotely well described by the youth word "lost"...

Once again I'm stumped when someone wants to upgrade the cheapo (Town & Country), which is economy... to economy... which is not recommended... so if I fly economy class, I shouldn't upgrade to economy class... but then what? Economy is already the lowest level, right? Should I just jump off a cliff with homemade wings?

... I guess some people here wish for that by now ;) ...

Could it be that you've possibly slipped in your own vocabulary?