I have revisited this thread after an interesting session on the courts. Occasionally, I will get a coach to work with and brush up on routines, work on consistency of shots. Let's face it - sometimes little bad habits on technique can work itself into your own game unwittingly. Note that my training before worked on singles play and I didnt really concentrate on doubles. Now, I stick mainly to doubles.
Yesterday, I worked with a coach who is specifically a doubles player at a professional level. Normally I have the same problem as the OP in wondering why I can't get that really nice smash technique. The coach tells me a few small things that make a big difference to my smash.
Preparation: a) he tells me to keep my elbow higher nearly to the level of my shoulder. My Dropping the elbow low means the windup needs to be bigger and greater chance of things going wrong - I.e. mistiming. I notice the OP's elbow is really quite low.
b) tells me my racquet head preparation is too closed - he wants me to prepare the racquet face more open (I.e. facing towards net). This puzzled me because the pronation movement is less. Again, shorter technique means less things go wrong. He tells me to think about how Jung Jae Sung prepares his racquet for the smash and then I immediately understand.
Striking the shuttle: a) he tells me my contact point is a bit low. Something I always need to remind myself.
Follow through: a) my shoulder is not rotating forward after hitting the shuttle. This is similar to the OP. The coach gets me to really rotate my shoulder much more after hitting the shuttle and to end up leaning forward at the end of the stroke.
b) lastly, I don't rotate my hips enough. Actually I used to do it a lot more in singles but sorta dropped out of doing it for doubles. Bad habits and laziness creeping in. We get the rotation going so my right leg and hip are forward after the stroke. You can see the OP doesn't really turn his lower body either so his right leg isn't forward after the shuttle is hit.
Results? Totally different sound. I get that nice huge explosive sound and have a nice big smile on my face

The shuttle travels much faster - my other training partners testified to this.
Contact point is a bit low - this will probably be the hardest part for me to change.
Very satisfied with the result.

I did around 130-150 smashes in sets of ten. Couldn't do more because I had some elbow tendinitis earlier this year so was being careful about protecting the elbow.