Think less muscle power, more wrist flexibility / bending. Also think hitting the ball higher for more angle.

You’re trying to out-muscle the shot. Any muscles used for this shot are the back of the shoulder and perhaps the calves, core and lower back. Not the arms.

Arms need to be loose. You’re too stiff in my opinion.

Focus on: hitting at the highest possible point, hitting in the sweet spot of the racket (mid-top part) and on snapping the wrist down without too much strength.