Selling new construction after receiving funding / Wohnriester - possible?

Madness ... you work and save for years on end for a cozy home, overcome numerous hurdles, are insanely excited about moving in and the time afterwards ... ... and then THAT

One (!) neighbor who ruins everything.

Is (at least) the approaching onset of the cold season a consolation, when the opportunities to sit outside will likely diminish and thus also the music playing?

What does your husband actually say about it? Does it bother him as well?

What does your husband actually say about it? Does it bother him that much too?

He is the one I feel most sorry for in the whole story.

Amazing ... you work and save for years for a cozy home, overcome numerous hurdles, are insanely excited about moving in and the time afterwards ...
... and then THIS

One (!) neighbor who ruins it all.

Is (at least for now) the approaching start of the cold season a consolation, when the chances to sit outside are likely to decrease and thus also the exposure to music?

We shouldn't immediately portray the neighbor as the villain. As Kati already wrote, it probably won't bother the average person. For example, I know it from Poland. There, there is no such thing as quiet hours, and it is often loud. At my mother-in-law's place, who lives in an apartment block, the neighbor upstairs knocks on the wall every Sunday, really EVERY Sunday for about 1-2 hours straight with schnitzels.

Personally, I wouldn't allow myself to be forbidden to play music in the garden on my own property. Of course, not with the bass turned up fully and not very loud, but why did I build it? Just to listen to music at home as in an apartment block? I think the "problem" lies somewhere in the middle. With Kati, because she is so sensitive, and with the neighbor who could possibly turn the music down a bit, not turn it off.

Judging all this from a distance is very difficult.

The problem is, you often can't control your surroundings. The nearest airport to us is 100km away, yet the planes roar over our garden. That sometimes bothers me a lot, but if I now call and ask if they can change the route by one degree to the left or right because of my garden, they'll think I'm crazy :D.

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We shouldn't immediately paint the neighbor as the villain.
...

That is by no means my intention either.
I abstractly see the neighbor more as "something" that motivates you to want to leave your newly created home. That can be any kind of thing or situation.
From the perspective of the previous owners of our house, the neighbors were also "bad" because they kept chickens and roosters. They are certainly not "bad" because of that. From the neighbors' point of view, the previous owners might have been the "villains." It is always a matter of perspective and feeling.
We get along well with all the neighbors. We do not, in our opinion, give any reason for the opposite. We don’t disturb anyone with noise and always greet nicely :) Recently, on a Sunday, I went over to ask if it would be bothersome to do some small work on our garden shed that involved hammering and drilling. After a few nails and 3 screws, everything was already done.
A lot can be achieved through communication.