Personally, I'm mostly using the app in the gym and progress by picking heavier kettlebells when it feels too light. Also, there's a setting in the profile for signed up users where you can select how complex exercises you want (three levels).

But I'm also thinking about how to add more progression even with a single-weight kettlebell at home and by not having to trust your own subjective feeling.

Edit: I'm also very open for ideas and feedback