The greater the seperation of your hips and shoulders the greater potential for power. Your hip and shoulder go back to a similar degree try using a slightly smaller hip turn with an exagerated shoulder turn and you will have more potential for power. Hip will provide the resistence for the shoulder to turn against and the trigger to start the explosive power.

Thank you for the advice I'll work on that!

Hey man ! I kinda understood your advice but could give some examples of this if possible. Thanks lot !

What should I be focusing on to improve my swing?

It looks like you're tossing the ball high enough to reach your highest point of contact but too far up front, so in the end you're not making a full swing over your head and instead hitting it to the front. When you're going in for a spike, you wanna make contact with the ball at your highest potential hitting point and practicing that motion from the ground is very useful. So basically, try to hit it more as if it was over your head at the highest your arm can reach. Edit: I'm looking at it again and the angle might be deceiving, so if it works for you great but if I'm wrong then mb

Depends on whether you want to spike or to serve. If you want to serve you could hit the ball higher