You can go with a feature phone or you can look at one of the small open-source Mobile OS. Firefox OS springs to mind.
No, you would not be able to run Fortnite on those.
You speak of false equivalences after equating Android with iOS... There are countless different Android distros. Most Android phones can be unlocked easily and/or are sold unlocked in certain markets. All software components are swappable. The OS can be run entirely Google-free, if you want. The Play Store is default, but optional. Sure, the mainstream hardware manufacturers sell preinstalled Android distros that require a bit of menu diving in order to enable free software installation, but there's little that's stopping you, as a customer.
I do not know of a functionality not available for development on a open source Mobile OS.
I agree that it costs time, effort and money to develop each functionality. I fail to see the choice between only iOS and only-Android. It is not mandated to choose only between these two.
Just like the ones you mentioned it’s not really an equivalent solution.