When recording a stretch, the app considers it as "Mastered" if I hold for the required time. The problem is that with flexibility, mastery should be defined by ROM and good form (controlled movement, steady deep breathing, etc.)

For example, someone who holds a butterfly stretch with knees touching the floor for 30 seconds has mastered the exercise more than someone who holds the stretch for 2 minutes but has their knees several inches off the ground.

My suggestion is that you continue to track time and also add an input field to track ROM/form. It could be a generic 1 to 3 slider or exercise specific. (E. g. for butterfly stretch, something like "Knees fist height above floor," "Knees just touching floor," and "Knees easily touching floor/not stretching.") The app would show a stretch as mastered only when you meet the required time AND achieve the perfect form level.

A difficulty/form metric could also be useful for strength exercises. For example, doing a chinup from a dead hang is much harder than doing a kipping or jumping chinup. Perhaps add the option to chose a "Strictness" level similar to selecting hand position or grip width.

Having one exercise with various levels of form mastery would be more user-friendly than making a separate exercise for every slight difference in range.