The developer requires a down payment to start drawing

It will be interesting once you start building and have to pay exorbitant prices for all services because you are tied to the company. Then a toilet bowl quickly costs 1000€ instead of 100€. You are rarely allowed to do any work yourself, and supplying your own materials is also prohibited.
I think an architect would be more sensible for you. With one, you can compare prices, negotiate, and pay the price that seems reasonable to you.

By the way, the catalog prices are generally absolutely not meaningful but rather bait offers. So if it says 350k there, it will more likely be 450k + additional costs.

A freelance computer scientist with a degree, who is not just a programming slave, will also be able to charge daily rates of €1000.

Our experts (usually with a degree in civil engineering or architecture) charge between €1,000 and €1,200 net as a daily rate. This matches the freelance architect.

However, at this amount, it is not comprehensible to me. I boldly claim that an architect who draws daily and knows everything inside out spends a maximum of 8 hours on such a project. That is already highly calculated. An hourly rate of about 60 euros results in just under 500 euros for me.

This is not bold, but either clueless or disrespectful towards the work of an architect. A good firm does not want such clients.

@hampshire Were you asked? :p

no, the incomprehension annoyed me.

so for you it’s normal to just pay 4,000 euros to every general contractor?

No, not for me. There is probably simply a wide range. Our first draft including the offer cost 0,-. Even after we didn’t build at the time, no invoice came afterwards, and the costs were not discussed beforehand.
But yes, I understand every general contractor who charges money for that. And again yes, I would also hesitate at 4,000 €.
But if it doesn’t fit with the contractor beforehand, and your feeling points to being ripped off, then try it with other general contractors...

no, the incomprehension irritated me.

Tsk tsk tsk....... just writing something without being personally asked by the OP...... I also think that's a big outrage