So you always ride in half seat like that? I mean, are you intending to be so far out of the saddle?
It looks like your upper leg is not doing enough of the work, same as maybe not enough core.
I think a lot of this might be due to you riding much too much off your feet, rather than off your whole leg.
Rather than just big stretches, try work on stabilizing the ankle. Stand on one leg, other leg kind of out behind you a bit, bend over and touch the ground iwht the opposite finger, come back up to standing, WITHOUT PUTTING THE OTHER LEG DOWN! Do 10 in a row, each side.
When you are good at that, pust a thick pad of rubbery foam under your foot and do the same excersize. you will feel all of the small muscles one uses to stabilize the balance (in foot and calf) moving to keep you balanced.