I'm by no means a Substack expert (I've been on it for less than a week) but I think you can just use both of them at the same time, and you don't need a huge audience.

The Notes feature is really nice on Substack, and it's a great way to connect with other writers.

Now, I can't really give any advice on how much money you can make on Substack because I don't have any paid subscribers, but I know some Medium writers who make $1k a month.