I've been making $500-1000/month on an iOS app for work time tracking – https://apps.apple.com/us/app/flexishift-work-hours-pay/id11...

When I started the project, the category seemed quite crowded already, but I couldn't find anything good for tracking my hours in a flextime arrangement. I had no iOS experience at that time (I was a C++ developer) and now I do iOS development as my full-time job too.