There must be tons of ad free iOS apps but generally most apps worth using cost something. Even just a small amount. A lovely app I use is Exoplanet[0]. Granted, it has an in-app purchase to make it ad free but that's like a cup of coffee.
You say that but they’re actually really hard to find because discoverability in the App Store is terrible. There is no way of filtering for them either.
By the way, that’s a very cool app - thank you for sharing!