This sounds interesting.
Although this use case is very infrequent. If this had a wider use case, like something used daily, that just might be be a viable product!
This is true, "habits" is too general of a topic. This is why Strava chose cycling as their first sport and expanded from there. Smart of you to focus on fasting, hot topic!
Wishing you all the best.
The worst part about a lesson like this is that it can take years to really get to this conclusion. I'm trying to ask myself "who is the buyer of this product". This is happening a lot in digital health where the payor isn't the end user.
This sounds interesting, I think the advantage here is that you've focused on 1 use case where I was focused on many habits. Keeping it focused on one niche like journaling makes more sense.