No building permit - splitter settlement, nature conservation, outer area

You need allies in the administration for something like that. Legally, nothing can be done, the situation is clear. Unless you become a farmer or something.

And how do you find them :D ? Make an appointment with the mayor?

In rural areas, usually through local clubs, volunteer fire departments, etc. You shouldn't just barge in but rather get involved long-term, for example at village festivals. That's how you make contacts. Of course, you can also simply make an appointment with the mayor and promote the project. However, it would be good if it wasn't just about your own project, but if you possibly represent a larger group of interested parties and highlight the benefits of a reclassification for the community.

I see little chance there and would recommend that you realize your desire to build a house on another plot of land. Even if you make contacts and invest your heart and soul, it can take decades. By then, you will be old and gray.