Ideas to find a niche for self-publishing
Here are the answers that I have since found myself...

1. Follow the content about self-publishing on YouTube and blogs
There are now hundreds of good posts that contain information that would otherwise take a long time to find out for yourself.
The best comprehensive guide I've found is The Freedom Shortcut by the Mikkelsen twins.
@JamesAltucher has also written some good articles on the introduction to self-publishing, for example this one.
2. Start with the things that really interest you, not with keyword research
3. Analyze these niches with tools or through intensive research
I use KdSpy to show me data about the books in a certain niche on all Amazon pages: pages, ratings, sales per month, price, sales rank, etc.
Other useful programs are Publisher Rocket and Helium 10 X-Ray.
Google's keyword planner is also very useful. It shows what people are searching for and the dynamic evolution of those searches.
4. So I made a list with 15 topics that really interest me
To be eligible, each of these subject areas should allow for the publication of a series of books. The topics should also be suitable for audio books.
5. Audiobooks are very profitable
Here, a large demand is faced with a still small supply. However, you must be a US or UK resident to join ACX. As a European, I can get audiobooks on Amazon through service providers like Xinxii.
6. Use other people's ideas and services!
From the outline to the writing of the content and the illustrations to the reading of the audiobook and the cover design, everything can be outsourced.
7. Look at the relationship between supply and demand for each results page on Amazon
If some self-publishers have made it to the first page and you don't have to compete with too many big names you have a good chance of earning money with a self-published book.
8. It might be worth considering other distributors as well
For example https://www.ingramspark.com/
9. Even books by unknown authors with unsuccessful marketing activities occasionally sell a copy
For example, my book from last year about simplifying life.
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