Look up the piece by pbs and by John Oliver. They also do a cocoa bean tracking system so they know where the beans are coming from. This is importa t to know that the beans are coming from the good farms and not from the bad ones. All that being said, they say they can't say slavery free due to how bad it is in that area. They can't guarantee that.
I think for me it's as simple as - do I go with the brand that seems to be doing something about it? Or go with the brand that misses it's own targets and is satisfied with the horrible status quo