Hello and welcome.
One piece double rear pannier may sound like it's easier to carry off the bike, but in reality it's not. It's a very awkward piece of luggage. Good single panniers have shoulder straps so you'll have your hands free. It's still not going to be comfortable, you'll probably have quite a bit of gear to carry, but it'll be a lot easier than trying to haul the single piece bag.
Re: brakes, is there something wrong with your current brakes?
--J