I think this is cool, and it might even scratch a writing itch I have with literally just taking notes. Your explanation of what motivated building this sounds a lot more like my experience with note taking than writing any other kind of prose.

One nit about the site I haven’t seen addressed yet: overall it’s pretty good on mobile (besides the front page screenshot being too tiny to understand; addressing that is a whole can of worms, but…) the header is breaking the signup button even on my very large phone, and stuff vertical alignment feels weird. You could fix the former, but I’d consider dropping “for free” (it’s a general assumption for most people walking up to a site unless they feel like they’re getting a hard sell) and adjusting styles to fit the content on one line. Definitely look at the vertical alignment though, because it’s gonna look weird regardless of wrapping.