YouTuber Tom Scott has just released a 16 episode series on making an app. (YouTube link behind the Spoiler.)
I haven't watched the series (although I will), but IMHO the first question you need to ask yourself is what it's going to cost on an ongoing basis and how you plan on paying for it.
For Slide Tilt Roll, the only ongoing costs were my Apple Developer ID* and a small website & domain name**, but something like Tom's failed messaging app is going to require some kind of server which will need to scale with the number of users along with ongoing support for users.
In addition, you want your revenue to match your costs. e.g. When I first came up with the idea for Slide Tilt Roll and the built-in level creator I had the idea of having a website forum which players could use to share levels - a forum which would cost money to be member. But after some thought I discarded that idea because the revenue model (paid membership) didn't match the expense model (server & bandwidth costs).
* I've let my Apple Developer ID lapse, so Slide Tilt Roll isn't on the App Store anymore. While the cost of the ID isn't much, the bigger problem is Apple requires apps to be updated. In my case that would mean upgrading to the latest MacOS, XCode & Swift and then rewriting my OpenGL fragment shader to Metal (in addition to any changes for the current version of Swift & iOS frameworks). Too much work for very little payback.
** I've since discovered it's nice to have a personal domain so I can create email IDs.