Sinking legs are a head position problem, not a kicking problem
The change
Look straight down at the bottom of the pool rather than forward. The body is a lever and the hips rise as the head drops.
Breathing
Rotate the whole body rather than lifting the head. Lifting the head drops the hips and undoes the whole thing every breath.
What not to do
Kick harder. A stronger kick on a badly positioned body costs enormous energy for very little speed and is why people find swimming exhausting.
