The back knee action is a result, not an action you can take.
I don't see your back knee as a problem anyway.
Your main issue is that your swing plane is a chop down. This is why your bat head is lagging behind your hands at the normal contact location, and you are fouling balls off.
Most instructors will tell you to throw your hands at the ball. This is the typical result.
After stride/coil, your bat head is sliding straight down off your shoulders instead of turning toward the catcher.
Keep your hands back longer, and use your back elbow to initially rotate the bat.