Are you aware of somsething like this ?
It seems to me the commonalities would be what you want to look for. I'd be looking for the most common tropes, and then if the story's the goal, build it up from there.
Incidentally this is the kind of thing LLMs are very good at. Have you tried just plugging them into ChatGPT/Bard?
But Claude 2 or GPT-4 out of the box can probably do a reasonable job of that question without your having to feed the text in.
Try giving it a short description or name of each creation myth, asking it to provide a synopsis and then drill into main points for each at a high level. Then ask it to compare them.
Then finally just ask your question about the average. You could start with that but probably will get better results if you break it down first.