Unpredictability of construction costs - Will the project fail because of it?

Yes, and I am a person who can assess you quite well regarding knowledge of house construction and the role of site manager. That is not an insult, but a fact. The two or the one who suggests that you should take on the site manager role does not have your best interests or your house construction in mind.

I want to emphasize that it can be fatal for laypeople!
In theory, it would reduce costs.
The prospective homeowners now have to resolve acute problems on several fronts and then reorganize themselves.
Unfortunately, you are already one foot deep in the swamp, but I think you know that.
Legal advice is really necessary here.
Also, the fact that the loan agreement is already signed and your apartment is sold puts you under time pressure!
When does the interest-free period of the loan end, that is, from when do 'penalty interest' charges apply?

Perhaps a mediation meeting with legal counsel on your side could help.
I certainly would not let myself be 'ripped off' by the general contractor!
Always remember, it is your money and you make the decisions about it.

That is of course true. The other builders or clients will not be any different. But I simply find the way they have become snotty and uncooperative by now inappropriate.

Dear hang_häuschen, even if it is currently emotionally bitter and disappointing for you and your family, you can be very, very glad that you will not be building with this construction company. It would have almost certainly led to much bigger and more costly problems if you had built with such a company. I see it as if you have just barely (with a lot of luck, I really mean that) gotten away. I also find your budget sufficient for a nice house build (...and garages I find ugly anyway. ;-) ). And as for "desperation," ;-) for most of us, "states of despair" are part of building a house, but they are temporary and in the end we all sit in our garden or on our terrace and drink a beer or juice or wine or coffee. ;-) All the best, please continue to let us be part of your project. A solution will be found and we will continue to help you as best we can.

Which bank are you financing through? Savings banks, cooperative banks, ... are probably quite accommodating. I've already had the talks with the cooperative bank. Direct banks probably not so much...

What does your bank say? Already called?

I can confirm here , a good friend dropped out at the same stage as you a few weeks ago. His house bank (savings bank) let him out of the contract for 750 euros on a financing volume of 300k. He had expected significantly more and was happy. But it went over the head of the superior at the main branch; the branch manager couldn't decide alone... fyi.

I have now looked into the BayBO. According to Art. 45, the room height of 2.40 m does not apply to living rooms in residential buildings of building classes 1 and 2. Am I wrong to find this applicable here?

The experts present here, please do not confuse with facts. Thank you.

I would currently really not aim for a single trade contract award. At best with a well-connected architect who has good connections to the most important companies. Everything else is currently difficult, as a small, unknown client is very far down on the priority list for the companies. On top of that, the inexperience is a very bad situation in such hard-to-plan circumstances as now.

Yes, this aspect, that as a self-inquirer you are basically "without recommendation" asking for an appointment from her Majesty the craftsman, is often underestimated ;-) I would also recommend approaching this second attempt with your own planning (self-paid architect) and having them prepare a proper tender (with individual lots, even if you prefer a general contractor).

He has shown solely by his statement that he can also postpone our project indefinitely..

This statement is wrong.
If no completion date is specified in the contract, it is to be fulfilled immediately.
If the company then does not comply, deadlines must be set.
And in the end, they are liable for damages.