You might want to explore using haptic feedback via the devices vibrator motor to prompt the user to breathe in and out.

One short pulse for breathe in, and a longer pulse to breathe out.

This would allow the user to perform the exercises with their eyes closed and the phone on their lap.

I know the visual functionality is core to your design, but might be a good idea to provide some optionality for users who just want to do the exercises without looking at their phone.