the horse looks a bit like he's laboring at that canter. Like he's not freed up in his shoulder. He has very little reach and appears tight, almost like he's cantering through molasses.
You would be better to drop your stirrup a bit, ( I know, sis's saddle). You do more work absorbing the motion with your upper body than perhaps you should. If you were looser in your hip, you'd be able to absorb the motion more there, with a stiller upper body. This might also help free up the horse a bit. It's almost like you are "pumping" him along.
Also, maybe let him have a bit more rein until he picks up and moves a bit better. He may be leary of the mid length you have becaseu it tightens, loosens, tightens, loosens, with each stride , creating a rythmic bumping on his mouth.