It's always best to start off small. The saying goes "if you market to everyone, then you market to no one". Once you get some learnings from the small niche and you better understand the audience's challenges and pain points, then it's always easier to go broader.

Here is a short video I did about the ideal target market when I was launching my #1 best-selling book: https://www.facebook.com/jeanginzburg/videos/444206866114742/ (I am not trying to plug the book, but the topic of this video is about the ideal target market)