What cost you the most nerves during the house construction?

That seems unprofessional to me and would be a reason for me to warn against such companies.

I would like to know why. We would have liked to visit the factories, but were not invited or encouraged to do so by anyone. To me, that seems like an indication of openness and transparency. What is unprofessional or disadvantageous about it, as long as one is not pressured to sign anything during the company visit?

What is untrustworthy or disadvantageous about it,

The tone makes the difference:

Companies don’t get back to you, they want you to visit the factory first,

It’s not an invitation to visit the factory (which would of course be a commendable gesture of transparency), but rather the opposite: an interested party is granted the "favor" of information / the offer only after making the effort to pilgrimage to the birthplace of the desired product. That is untrustworthy, because presumably the calculation behind it is to use three psychological effects: 1. Emotion: on the way there, anticipation about the unveiling builds up, the "event" is "hyped"; 2. the factory visit at the first manufacturer creates an anchoring effect, which strengthens that of a first model home visit at the same manufacturer (or overwrites a first model home visit at another manufacturer); 3. the inhibition to make "sunk costs" of this effort causes the manufacturer to receive, relatively speaking, bonus points in an otherwise roughly equal situation, making them the winner. Whoever uses the factory visit manipulatively as "no show, no treat" deserves, in my opinion, a red card. Testing the honesty of an interest is legitimate in itself, after all, calculating an offer costs time and money – but this should also be possible for a sales consultant in other ways.

We are not there yet either, but what stresses me the most is the waiting. Almost 1 year for the development of the construction site, then waiting forever for the surveying, 3 months from the notary appointment to the conveyance, waiting for suitable weather. Waiting for [Küchenbauer2] to bother to provide an offer (especially annoying because I have kept [Küchenbauer1] on hold because of this...)

Waiting for kitchen fitter2 to bother to make an offer (especially annoying because I kept kitchen fitter1 hanging because of this...)

Are these two possibly waiting for each other because they are getting a project price from the same wholesaler?

I rather suspect not. As far as I can see, they have different brands for the furniture. The problem is probably that Küchenbauer2 has an employee who doesn't really take care of it properly. He somehow missed the first appointment. Which actually means the decision for this company has already been made...

The problem, I think, is rather that Küchenbauer2 has an employee who doesn't really take care of it. He somehow also bungled the first appointment. Which basically means the decision for this company has already been made...

With that, in terms of the title question, you yourself would be the biggest troublemaker because you are still keeping Küchenbauer1 hanging ;-)