I agree with some of ZT's comments. Perhaps the stiffness that appears in your video is a result of just trying to concentrate on everything a little too hard.
As a piano player I have to do some scales to a metronome sometimes for my technique but I also need to sing some songs in the dark from my soul sometimes - and that helps my technique too.
Swimming for me is the same.
So try some soul lengths. (or even better long open water somewhere pretty).