The main issue I see is the swing path and what to do with the ball. The TS shot should not be approaching the ball horizontally and slapping it. Instead, should come under it and brush up.
The yellow line connects the handle to the tip - the future multi-slam winner in the last picture has the racket aligned horizontally in the slot position than the previous slam winners on the left

Since you swing slower than pro players I would think your angle of attack should be steeper than them. imo, the blue line should be your angle of attack to brush up the back of the ball. The path should be more golf like swing than a horizontal slap. The racket face appears a lot more open than others (grip or supination/pronation/esr fine tuning)
It's the exact same issue with your forehand as well.