Launched my MVP! Now I have 5/3/1 program encoded there, I can track my workouts, I have history, a rest timer. I start to use it to track my own workouts.
There're things I still miss a lot - progress graphs on main lifts, I can't change the workouts in any way (they all hardcoded), everything is stored on a device only (no cloud storage).
I created a TODO list, adding ideas there, and keep working on the app.