I do mobile app development and I love it. At work I do that and a little bit of backend and web frontend development and I just don't enjoy those as much. There's more overhead in getting things done there, whereas with iOS development, you have a more constrained space where there's generally just one way to do things, so you spend more time thinking about the problem you are solving vs. dealing with technology issues. Yes, there are still things you have to figure out on iOS that don't have the greatest documentation, but I find it's much more straightforward than the other spaces I mentioned.
If I could write whatever I wanted to on whatever platform, I'd do what I'm doing now as a side project: a flash card app on iOS and macOS. [1] I'm finding it lets me deal with the entire "stack" of interesting product problems: technology, user experience design, solving a real user problem, and even marketing. I'm not treating it as a serious job, so it is mostly fun to work on, although I am getting close to releasing it, so it can be a slog. (This reminds me of what I read about the author of Stardew Valley and how after working on his game for several years, he did find it difficult to keep working on it, which is a problem you really can't get away from.)
[1] See https://www.ussherpress.com/blog/ for some posts about it if you are curious