There was (still is, though the website is now dead) a really cool website/app that a fmr Air Force soldier made called AFI SWiM[0] that extracted 'Shall, Will and Must' references from Air Force publications which blew up in popularity among the target demo. The dev recently(ish) wrote a blog post[1] how it surprised him to see the app[2] had gotten around ~20k downloads, though I am unsure if the app was always priced at $1.99 (which would be just under $40k).
I like the idea of developing a problem solver for a niche market, though finding and monetizing it definitely is pretty tough.
[0] https://web.archive.org/web/20220601160702/https://swim.afie...
[1] https://willswire.com/blog/chat-afi/
[2] https://apps.apple.com/us/app/afi-explorer/id1564964107
the github repo: https://github.com/willswire/afi-swim