Good description Qwerty, the only other thing I can add is when the kick ends e.g hits the target, your front foot left in this case, should be facing almost behind you as pivot left on the ball of your foot and turn your hips in same direction, this should be 1 fluid motion through the target, instead of hitting it and pulling away when you do so. As your quite a big fella you should be able to do some good damage when you learn the right technique.
The only other thing is you'll know when it's about right as you'll notice more of a quiet thud more than a loud slap when the kick hits the pads.
Good examples are Buakaw por pramuk, Gilbert Yvel, Mirko Cro Cop to name a few.
This is true if your kicking a banana bag or a human body but usually when your kicking a heavybag the circumference of it is a lot bigger than a body so even a perfectly placed kick will have some foot contact and make a pop instead of a thud. The bag I usually kick is so big that if I kick it to make a thud per say, I would have knee contact lol