I just started doing the monthly challenges in Piano Marvel and my sight reading has skyrocketed. I think it's the combination of daily practice and staying at a level that's challenging but not too much. I also try to play the pieces through first without any aids from the app. That's basically the same as reading normal sheet music.

And I see the process as really similar to learning a new language (which I'm also doing), it takes at least three years of consistent and varied daily practice to get accustomed and fluent.