This is close to ideal, but could certainly use some design improvements. Hopefully some of these issues will be taken care of.
You can’t see which rep you are on in the middle of a set. For me, I tend to get mixed up during a set, and not knowing how far I am is not good for motivation.
The timer stops when the app is put in the background. This messes with recovery timing if you switch to a new app.
The audio choices are odd, but are tolerable.
This is nit picky, but a timer should use a mono spaced font. The width of the numbers on this timer pulsates back and forth as the sub seconds race along.
Thanks for putting this together! Please implement some of these changes, especially the current rep number! 😊