Is the construction project affordable? 570 k€ loan with a 5300 € salary

That's true. But it usually isn't about the money.

It sounds absurd when shortened like this.
Starting every day happily in your own four walls, being grateful to play with the children in the garden, living in a stable, good, and clear neighborhood, having the good feeling to have your own retreat space available... A lot happens while you're financing the property. The right home for your own life can contribute a lot to quality of life – whatever that exactly may currently be.

You talk about six-figure wishes. Why only six-figure? Why not eight-figure? Why not two-figure? Once you reach a basic level, which many of us have for a happier life, it doesn't really matter much. If it works, it works; if not, it doesn't matter. You can always have wishes. That’s a good thing. If I buy a vineyard with an olive grove in Italy, I’m happy. If I don’t find one, I’m not a bit unhappier. I don’t make my life happiness dependent on luxury demands.

And about retirement – as if that’s the fixed big turning point. From that point on, there will be money (however much or little) without having to work in return. If work has long before already cost quality of life, then the question of a turning point makes sense earlier. (Always under the premise: lacking money in the sense that it’s no longer enough for food and housing is a tyrant that makes you unhappy. Perceived setbacks in lifestyle only make unhappy if you cannot let go of standards. Lots of money is pleasant and very welcome but not important for a content life.

And I have to say this relaxed feeling is already worth a lot. We don’t both have to work full-time to cover the mortgage.

It clearly says there is no garage included! To your "much too cheap" I counter the following example calculation:

150sqm x 2200 € = 330,000
10% incidental costs = 33,000
Basement = approx. 70,000
Photovoltaics = approx. 15,000
Outdoor facilities = approx. 25,000
Carport = approx. 10,000

Subtotal = approx. 483,000,-

The original poster states a total of 618,000, meaning for the plot + development roughly 135,000 euros remain... with only 315sqm that would be roughly 400 €/sqm.
That is more than realistic and anything but "much too cheap."
Even if we assumed 2500/sqm that would still fit (if the price per sqm for the plot is around 280 euros), especially since the OP even gives relatively concrete itemization costs on top!

But it's all OFFTOPIC - SORRY! :D

So I have about 80 net salaries in credit. And when everything that needs to be paid is paid, including a reserve for vacation and car, then I have about 10% of the net as a free amount, with which I can start doing whatever I want. That is too little for me to be relaxed, to be honest. There are simply too many "holes" where I could put it: the famous special repayment, the parking space still needs to be paved, my parents supported me, but they want that back... not to mention any kind of "luxury" (I would really like to learn to ride a motorcycle, for example). That will not be possible without guilt for a very long time. And the salary increases? Forget about it... Inflation immediately eats that up. This year: +1.3%, that already went directly to the increased energy costs thanks to the CO2 price (gas, electricity, fuel).