Renew windows / Is the offer fixed or negotiable?

Then you have already given yourself the answer. I am always surprised when people ask questions and then get snappy in the end, accusing someone who knows better from experience of having no clue... My friend, I have been working professionally with assembly companies from Eastern Europe for more than 12 years. It ranges from "I don't have to complain anymore" to "we're getting a German company because nothing fits here at all..." Buy the windows in Poland. As a rule, it will go well. If it doesn't go well... you have a problem. That's all I said, nothing more and nothing less. Whether that is worth 1,500 EUR less to you in the end, you have to decide for yourself.

Sorry, but I will not get snappy just because I am correcting you. I don't know what your experience with "assembly companies from Poland" and "I should buy in Poland" has to do with windows from a German, established window manufacturer, but okay. You are kind of getting carried away. Especially since German companies now also operate locations in Poland. But this should not turn into Germany vs. Poland here. And I just wanted to clarify that the manufacturer is not the contact person for warranty claims.

I don't know what your experience with "assembly companies from Poland" is

I can at least tell you that Poles belong to the most sought-after craftsmen because the quality of their work is excellent (good training). If you are building new, it is quite possible that the German construction company also sources the windows including installers from Poland. Nothing special. Although, of course, you have a German contracting partner there.

Buy the windows in Poland. As a rule, it will go well.

If it doesn’t go well... you have a problem.

That’s how it is.

We never had a mold problem in previous apartments either. I think proper ventilation counters that. So far, we have never had a mold problem in any apartment due to proper ventilation.

You didn’t have to ventilate in that house either. The mold problems after window replacement occupy many owners every day and even more tenants and landlords (and thus experts and courts). Believe me. Keep an eye on these issues.
And as I said, a ventilation concept is not a wish list, but an obligation. You want a proper installation. Then you also want a prescribed concept.

The gas consumption for 190 sqm amounts to about 17,000 kWh, which also includes the hot water.
Our wood consumption is also very low.

LOL. That is not low. That’s about 2,000€ heating costs somewhere. Physics - as stated. You can’t heat cheaply with that.
If I convert my house (which is a bit smaller) to 190 sqm, then I heat for 500-550€ including hot water (22 degrees room temperature). That’s what I mean by subjective.

You didn't have to ventilate in that house either. The mold problems after window replacement affect many owners and even more tenants and landlords (and thus experts and courts) every day. Believe me. Keep an eye on these issues.

We will. This is not our only/first house. Ventilating is not a foreign word to us.

LOL. That is not small. That's somewhere around €2,000 in heating costs. Physics—as written. You can't heat cheaply there. If I convert my house (which is somewhat smaller) to 190 sqm, then I heat for €500-550 including hot water (22 degrees room temperature). That's what I mean by subjective.

So if you heat a 25-year-old house with 190 sqm for €550/year including hot water with gas, that is astonishing and probably unique. The average gas consumption for a 190 sqm house including hot water from 2000 is above our consumption.

But that also misses the main point.

We want to renew the windows in a timber frame house. 12 windows + 1 balcony door.
The windows are 25 years old. Apparently, there were significant errors during installation; the windows are not properly sealed, fastened, etc.
Accordingly, it is quite noisy.
Due to the close proximity to a street, you hear every passing vehicle. [...]
According to Oknoplast, soundproof glass is not possible with muntins; with the German supplier, this would be no problem.
[ / ] a 25-year-old house

So the windows and the house are the same age, presumably installed with the house and factory-made in the prefab house factory. If they are still poorly installed, this speaks for a cheap manufacturer (of the house). Wood moves, also in prefabricated wall panels. Find out what the cause is. And make use of the options to adjust the windows and, if necessary, replace their seals. Windows (what material are they actually made of in your case?) do not necessarily have to be "automatically" removed after 25 years. By the way, windows that have already been installed can also be laminated if they no longer look neat. Whether muntins are a thermal bridge depends on their installation position, not on the brand (or the nationality of the manufacturer).

Forget about an online dealer with "German installers," as those are often just some random handymen from the craftsmen's market. I would prefer a Polish window manufacturer over a German general service craftsman. As a former window manufacturer (aluminum), I can sadly attest that even with the best products, you can produce first-class complaint cases – it "suffices" if they are bungled into the wall openings by disinterested bunglers. The number of unscrupulous characters operating in this field is beyond belief. So you have a high chance of literally "throwing money out the window" on botched improvements. From my point of view, the question "where should I just buy some random new windows" is the worst possible approach.

Sorry to all the “older” questioners that I answer a younger question first here with "last in, first out," currently I have to manage my time very strictly.