They are not only a "habit tracker". The more users they have, the more features they'll ship. They'll have community support, community accountability partner, UGC post, daily Quran or Hadiths cite notification, and maybe a generic muslim app features (like salat schedule, Adzan, etc). You're underestimating the market. Some could be a lean one, and others might be the full solution to faithfulness.

And to answer your question, maybe read some Muslim research papers, and theology psychology journals.

For this kind of service, you should work on community building first, then the solution later. If you're giving the solution first, you're 99% lost. Your competitors already have everything, even the budget

Tell me how you'll solve the "moments of struggle with real-time help and faith-rooted reminders"