A piece of my experience. My legs were not really sinking but they were (including the pelvis) slightly to low for my taste.
For me the issue was lake of hips rotation relatively to the torso, so I practice at letting go(rotatte) the pelvis hips as I swam with some speed. Over doing it to some extend helps to findspot the tensions.
Then relaxation + speed got the hips to sync properly on their own (and the kicks).
Still working on it to install it for good but the feel is there, not my biggest focus at the moment, proper breathing is.