It sounds like you're already doing well. Circles, serpentines, renvers, travers, getting her to bend her nose back to the girth (or as far as she will go at first) while you are on the ground...those will all help her achieve more flexibility over time. Work firmly through her balkiness. Ride with quiet hands and a quiet seat (especially if she is bucking at the canter, not too much driving seat). I'd get her working more willingly in the walk and trot before attempting much canter work. I'm not a big fan of hackamores in general, especially if she is just generally stiff and balky (they seem to work better the lighter and more sensitive the horse is), but if she works well in it, so be it. She sounds like the type who needs some alone time with a single, confident rider who's going to be very black and white with her.