It looks like this describes the same problem How to ignore subfolder and can be solved by https://getkirby.com/docs/developer-guide/advanced/routing#omitting-the-blog-folder-in-urls.
As the categories are unknown and I would have to search all directories for a unique match, I think it’s better to change the directory structure.
If someone else has an idea, I’d be really happy.
Thanks again
Tobias