Perhaps instead of allowing full control you could have different modes? I like using the 4-7-8 Method[1] but there are lots of different breath exercises.
I am quite new to breathing exercises but I also like the 4-7-8 exercise a lot! I was thinking about implementing the most known exercises and then have the option to create your own exercises. A lot of people have been pointing out that the app was way too slow and that it was difficult for them to keep up... I think that maybe if they can increase progressively the duration with custom exercises they can adapt better (it's just a theory)