Renew windows / Is the offer fixed or negotiable?

If these are reputable companies, I wouldn't hesitate long at that price. It's really cheap. You can watch the installation to see if it is done properly. If you do some research beforehand, you can tell if it's heading in the right direction. You're only 100% sure if you do it yourself and can.... But that's always the case with everything.

So if you heat a 25-year-old house with 190sqm for 550 euros/year including hot water via gas, then that is amazing and probably unique.

No. Year of construction 1965. But renovated. Therefore not unique, but cheap. Little and cheap are not the case for you. That is what I meant. Maybe good for you, but not in absolute numbers.

If they are still poorly installed, that speaks for a cheap manufacturer (of the house).

The windows were installed during the house construction by a local master carpenter, who still exists today (in the second generation).
However, the installation must have been faulty, because the windows are not properly sealed. There are also local craftsmen who do shoddy work.
At most, one could remove the windows, reseal them, and reinstall them properly. But then I might as well replace the windows.

Whether muntins are a thermal bridge depends on their installation position, not on the brand (or the nationality of the manufacturer).

I just find it strange that Oknoplast says that soundproof glass with internal muntins is not possible, whereas TMP says it’s not a problem.

Forget about an online retailer with "German installers," there are likely some random guys from the craftsmen market involved.

I only checked the online retailers to see what options are available.

If these are reputable companies, I wouldn’t hesitate at that price.

As I said, both come from the region and have consistently good ratings/reviews as window manufacturers. These are not backyard fitters.
So I interpret that the price is "fair."

I'm only surprised that Oknoplast says that soundproof glass with internal muntins is not possible, whereas TMP says it wouldn't be a problem.

It could be due to a language barrier or the exact wording of the inquiry. EVERY glass manufacturer installs internal muntins, as they are basically irrelevant for soundproofing. It is not possible to not get such panes from all possible glass manufacturers in Europe.
Muntins dividing the glass are a completely different matter.
Or they simply have no desire to take the order.

I am only surprised that Oknoplast says that soundproof glass with internal muntins is not possible, whereas with TMP it would not be a problem.

One salesperson looks at the sales, the other at fewer complaints. Internal muntins are of course also thermal and sound bridges. Personally, I would not take the fashion of the time 1:1 when replacing windows.

I personally would not take the fashion of the time 1:1 when replacing windows.

There is no alternative for the object, as only the windows on the 1st floor are being replaced and accordingly the muntins on the ground floor are retained. To maintain the overall visual impression, the muntins must be put back in.