I am happy with Emacs / GitHub which are both free and very effective.

Having said that, I have paid for writing/note taking tools in the past. I bought Ulysses which is a Markdown note taking/writing app. It's available for Mac and iOS. That was a one-off payment of around $30 for the Mac app. I loved it, used it for years, but moved away when they introduced a subscription model. I do still occasionally use my paid for app though.

With regards your $5/$10 a month - personally I would not pay it. I know that because I was paying around $7 a month for a private GitHub repo and that cost annoyed me. It doesn't sound a lot, but over years it does add up.

So, I guess, are the new features you are going to build into your app worth 5/10 bucks a month?