I've usually got a couple iPhones laying around. Every time you get a new plan here, they allow you to get a new phone and you usually just keep the old.
My current phone is an iPhone X, but I use an iPhone 6 plus with my Bebop. This way, I have a dedicated display/controller that only connects to the skycontroller or drone. Battery last fine, but the only thing is is doing is streaming video via wifi. Since it's no longer looking for networks and such, it probably helps the battery last.
I also have an old iPad 2 that I use with the Bebop. Battery also last fine on that old warrior, but again, all it is doing is communicating with the Skycontroller/Bebop.
Its a toss-up as to which one I will use at any particular time. If I'm going somewhere I'll probably just take the iphone 6, small and easy to carry. The ipad I prefer when I want a nice, large screen. Only problem is it's not the "retina display" the iphone 6 has, so the image isn't quite as clear. Each has it's time and place.
Maybe get a new phone and dedicate your old one to just "drone patrol"?