Financing monthly installment €2500 with 40 years term

And who is supposed to pay off the house then?
With the stated rate of €2546, we are talking about 3.8% interest and 1% repayment!!!
Total costs after (theoretical) 40 years would then be €1.232 million!!!
You’ll have to add a bit more...

Or try to get the land through a family loan (not a gift) and then pay with "equity". The land is given as collateral and thus the interest should be correspondingly lower.

about family loan (not a gift)

Does the family also want to wait 40 years for repayment? One would rather want to repay at 5%. Everything else may not be called a gift, but somehow it is one.

You have to step it up a notch...

You can do it later as well. It's called a repayment rate change. Close now with 3% repayment and secure a favorable interest rate. Then immediately adjust to 1% (not every bank offers 1%, so be careful). And as soon as it fits permanently, increase again. Or alternatively, plan special repayments. But be careful there too, as these often end up being spent on vacations or similar. This is more the rule than the exception.

Ancillary construction costs, such as construction electricity, construction water, earthworks, and the base slab, are included in the construction company's offer. They also work with a fixed price guarantee until completion.

I strongly recommend so that you can check whether everything is really included. Earthworks often cover only 30 cm; if more is necessary, this is subject to extra charges, etc.

We don’t want to increase the repayment rate yet, as we want to stick to our desired rate. After all, we still want to be able to live.

Did you not live on a salary before? I’m sure that even with a higher rate, you can still go on skiing holidays, etc. Please keep a household budget book or at least a comprehensive listing of all your expenses. You will never be able to increase the rate as easily as you can now. Once a certain standard of living (vacations, cars, hobbies) has set in your mind, you won’t be able to reduce it again. 1% repayment is simply too little.

Incidental construction costs such as construction electricity, construction water, earthworks, and foundation slab are included in the builder’s offer. In addition, they work with a fixed price guarantee until completion.

The devil is always in the details here. For me, construction electricity is included too, except for the thousand (hopefully only one) that the hotboy charges for heating the screed. What about surveying? Soil report? Construction road? Crane site? Porta potty? And so on. What about tiles and floor coverings? Those are always limited to a certain budget, e.g. tiles up to €30/sqm. Then the risk is on you if the €30 tile is actually sold for €50.

Please make a household budget or at least a comprehensive list of all expenses you have.

Also a good point, and there is a valuable checklist for this at the beginning of this forum section: