A way to cancel a schedule would be nice.

Maybe schedule() could return some kind of identifier value, and then you could pass that to a function that would cancel it. Haven't really thought of a full-suggestion for it, just that something to do it would be nice to have.

I wouldn't personally be in favour of skipping adding functinoality that can be easily added in-app

well, you can technically do that yourself by checking some variable when running a scheduled function to continue or not, right?

passing the callback ID as an argument