You chose your schedule.
The likelihood is you’ll get turned down to Brown - so what if you take AB and still get rejected.
If you take your current class and get rejected you’ll assume it’s why but you’ll have no idea why.
That you don’t want STEM is not the reason to avoid AP Calc.
That it’s too hard for you, above your capabilities is the reason.
There’s lots of great schools out there, Brown and otherwise.
There are no dream schools. Many great ones. And don’t panic on things you don’t know about / you are panicked thinking this class would hurt you but you don’t know - so don’t panic.
Good luck.