I have very limited experience with it, but my articles get more views on Substack than on Medium and I only have 65 free subscribers. I've also heard from others that it's much easier to reach a bigger audience on Substack compared to Medium.

Everything I've published on Substack is free for now, but some people post 75% free content and 25% paid content.

As far as niche goes, I mentioned before that you can still focus on self-improvement/mindset articles but write it from the viewpoint of being an athlete, so that can set you apart from most writers. That's just a suggestion, but I'm sure you can think of other things too.

Substack has a lot of potential, but it's something that will take months or even up to a year to build, so you really need to enjoy the process or it will not be fun.

If you want to learn more about how it works, you can just create an account and check out other writers and it will give you a better feel for the platform.