My basic advice would be to loosen up a bit. Despite the knee bend, your serve seems to mostly revolve around your shoulder. Try to get the momentum moving more forward.
Also, all these movements- thrusting upwith the legs, swinging up and through the ball, allowing the wrist to flex, bending at the waist, and so on, all need to come together so their force combines at exactly the point of striking the ball.
Sometimes, with the nice, modern rackets we have, it is difficult to sense when you have achieved this unity of forces. So, here is my other suggestion. Go out (like to a thrift shop) and find an old wooden racket and practice with that for a while (when no one is looking). Unless your form/timing is nearly perfect, serving with it will be a miserable experience. When you can, finally, start serving them in at the same speed as you do now, then switch back to the modern racket. You may find you are getting a bit more pop on the ball.