I don't think that a back button at the top of the screen is very usable. I think it kinda makes sense to have it there, but it's not something you should be us

I don't think that a back button at the top of the screen is very usable. I think it kinda makes sense to have it there, but it's not something you should be using as the primary back input. I don't know what you mean by "the back gesture itself only works within your app". I assume there are cases in Android where one app took you to another app and you can do the back gesture to go back to the app you came from? Anyway, the iOS model is that the back gesture just navigates within the app, and there are other features for navigating between apps. I won't make an argument about which is better, but I don't think iOS's model is obviously much worse.