A designer I know IRL helped me with the design. I also had a professional copy writer I know IRL help me with the German texts (not the English ones, though). Paid both of them, of course, but that was not too expensive, since they only helped, and I did a lot on my own before and after their involvment.

The Artist is Irina Linuza. Found her on upwork, and was very happy with her way of working from the start: https://www.upwork.com/o/profiles/users/_~01678d123d582fc42e... That was more expensive, since the illustrations are so detailed. Simpler ones would have been cheaper, of course, but that's not what I wanted.