It's amazing to read what people think is fair use. Have a read of the wikpedia article, it's definitely not as straight forward as adding a slight variation and then I'm good.

If that was the case all of the following would be fair use (hint they typically are not).

- making a movie from a book (I mean it's a pretty big variation) - making a sequel/prequel to a book, movie using the characters of the book - making new comic stories using Mickey Mouse/Batman/Superman... - Making merchandise using redrawn characters from comic books... ...

Also it's important to note that one of the important aspects to judge fair use is commercial interest, i.e. what is allowed for an individual privately, might very well be copyright infringement for a company (like Midjourney) who sells a commercial product.