Have you considered bringing the «Sign in / Sign up» after this experience?

You put a lot of thoughts into the onboarding flow and great ideas, but as a user, I want to be able to try an app and judge its value before creating (one more) account (eventually leading to more spam Newsletters) and giving personal information.

I know this might be technically challenging to manage user data without an account but in the case of your app I see the value of having an account for cross device experience (and cloud storage?).

What if you remove the signup form from the screen and go for «start using» (and «I already have an accout»). I use the app for a couple of days, create content, love it. Then, you trigger a notification : «Create an account to do X, Y and Z».

I would be intetested to know if such kind of experience would increase the activation and usage rate of a product.