You can write on any topic. I’d suggest you check out what others have done with the same subject matter, and ponder what’s your take. A good name is significant, Check out my post on names/trademark.
I think those who have achieved “millionaire status” had “names” to begin with. It is not easy to build a following on Substack unless you have some “draw.”
I’m halfway to my goal in terms of earnings, but I put a lot of time into it, and am working to balance with putting my own (fiction) writing first. I’m not trying to be huge :) I’ll be content with “enough.”
Check out my other pieces (and my newsletter!) and let me know if you have other questions… time for another article, I think!