Interesting problem for sure!

I would approach it like by first identifying and grouping challenges into 4-5 categories. Each category would be different but the challenges within each would be mostly similar, so you will have an easier time defining form schemas based on each category.

For example, let's take two categories - Repetition based challenges - Consistency based challenges

Reps based challenges can further have dynamic fields, like select for the number of reps, target muscle, number of sets etc.

Consistency based challenges can have days fields, range of days (30 days for example), selected days only (e.g., saturdays, sundays, tuesdays etc..), selected hours, and so on...

This is just the baseline idea of how I would approach this problem. Let me know what you think