First, I have always assiduously tried to not fall into the trap of being a niche writer. I have found being eclectic very beneficial. I think it goes back to my time as a highly successful freelance copywriter in the 1980s, continuously having to write about a very wide variety of subjects, from Indian restaurants to NASA extra terrestrial technological products, like lubricants that have to work in a vacuum.
KDP? Well I have written several pieces here about how to go about self publishing on KDP. I highly advise self publishing as it places you in control of the whole process, not just the writing. I have also loved acquiring new skills such as creating my own paperback book covers on Canva with free images from the likes of Unsplash, Pexels, Pixabay and even Canva itself. If you do self publish, you can do it very economically. I only spend about 35 dollars a book for formatting as I find that too complicated and time consuming.
As a self publisher, you stand to earn more as the royalties go directly to you, not some publisher middleman. A couple of things to be aware of; it can be very hard to earn from ebooks only. Also, the best selling genres in paperbacks are fiction, thrillers and mystery and suspense sort of things. Poetry and childrens books are very specialised and not big sellers, as a rule.
Personally I write short stories, novelas if you prefer, mysterious, action, dramas. I have also tried poetry, not a great seller, and my Katie books, again, not a big seller, at least not as ebooks. Now trying those in paperback to see how they do.
Katie? Just some whacky idea about a writer with supernatural life experiences which she writes stories about.
Hope all of that is a help. Thank you for your interest.