I don’t have a straight up list to offer you, but https://lemmy.dbzer0.com/u/db0 has made a software for looking up instances with high amount of users but low amount of posts called lemmy-overseer. https://github.com/db0/lemmy-overseer

On github you can find link to their running instance with API documentation.

I have not personally used it, so this is as far as I can point you :)