Construction delay of 2.5 months due to faulty window delivery!

Honestly, I think there is nothing to be done. Console yourself with the fact that you don't have to distribute any profit to a general contractor.

That's exactly why you have to calculate a proper buffer. You surely did that. So make the best of it.

A proper buffer and goods from the internet are contradictory to me. Just a gut feeling.

Having a proper buffer and goods from the internet is contradictory to me. Purely gut feeling

That was not meant for that either. Whoever wants to be smarter than everyone else has to pay for irregularities. Nevertheless, I wish the OP that he gets through this bottleneck well.

The first window delivery arrived on time as agreed in the contract.
However, the delivery came via a third-party provider (Poland) and showed various, partly significant, defects.
To ensure long-term satisfaction with the windows, it was agreed that the windows would be produced anew in Germany. This took 4-5 weeks

That sounds - if not very briefly described - like a stroke of luck, especially with full prepayment: without involving a court, the supplier agrees to a re-delivery of custom-made goods.

I gave the company a deadline of 8 days yesterday.

Which company exactly: as I understand it, one of the trades involved who have to fit into the schedule cascade?

In my opinion, eight days is not a reasonable deadline, but rather a grace period if there is already a proper delay (but I am not a lawyer).

Thus, all trades were pushed back.
You can surely imagine that they were not happy about that, especially in times of skilled worker shortages and full order books.

Now I am naturally at the mercy of the trades schedule-wise.
They always squeeze me in wherever possible.

So, firstly, they are "only" angry, no one has dropped out on you; and secondly, none of them are demanding damages or higher prices now? – you can hardly come out with less harm.

So we are currently at €6000.

As you know, I ran an aluminum window manufacturer’s company. In a high-price area, too. Even we would hardly have managed to offer your project more than six grand more expensive than local plastic window manufacturers – prices from online dealers cannot actually be worlds cheaper for the same quality either; it’s somewhat different with stock items (tilt-turn 118/148 or front door 110/210, white).

However, the delivery came via a third-party provider (Poland) and showed various, partly significant, defects.

Tell me more details, gladly also via PM/email. The Slovaks are a little better still, but the Poles actually work properly as well.

Yes, but here we have individual awarding combined with goods from the internet. The OP can be glad that a craftsman is installing it.

We manufactured and installed, so I know that warranty trouble almost always occurs during the installation phase. I consider it highly unlikely that disputes are settled amicably if installer and manufacturer are not connected. That’s why you should basically frame and marry installers who commit to third-party products. I wouldn’t have done it, and I don’t know anyone who does.