Welcome to Epic Ashwin!
The snowboarding piece of Epicski is a bit of a backwater but we're glad to help where we can. You're observations about using the upper body too much are accurate. All we need to do is put them into context a bit and figure out changes that can help get you more performance. The good news is that your heel side is "sitting" enough, your toe side turns get some nice edge angles and board bending and you've got great speed and directional control. The bad news is that I've got lots of options for things you can try with your riding.
Here's what I see:
Pic#1 - Finishing your heel side your shoulders are out of alignment with the board (open to the direction of travel), your legs are extended, your hips are not flexed and your ankles are neutral to slightly open.
Pic#2 - This is an example of your toe side initiation - bending at the hips to get the weight shifted to toe side edge so that you can tip the board onto the new edge.
Pic#3 - Here's a nice shot of the board bending on a toe side turn.
Pic#4 - See how straight the front leg is? The front shoulder is so open that the front hand is over the heel side edge.
Pic#5 - Here's a view of the sitting position for a heel side turn. It's not bad. You could do more, but you could also get more out of what you already have. Here you can also see how the shoulders opening aren't pulling the nose of the board into the new turn.
Lesson#1 - Start your turns with an edge change on your front foot first to twist the board
Try this exercise at home. Stand in a normal snowboard stance with shoulders lined up with where the board would be. Now rotate your front shoulder (and hips) to open like the pic above while leaving your front foot flat on the floor. You will probably end up with more weight on your back foot. Reset. Now shift your hips to be over the front foot (zipper over the toes) before you try to rotate your shoulder again. Ouch! The pressure in your knee isn't fun. Reset. Shift your hips forward again, but this time lift the toes of your front foot as you rotate open. Aha!
Lesson#2 - Stance width
Stance is a matter of personal preference. You don't say anything about your stance history, but if you started 10 years ago I'm guessing you started riding in a normal stance and switched to duck later on. I'm going to suggest that you try some stance changes not for the purpose of making a permanent change, but to facilitate learning some new movements. In Pic3 we can see your back foot underneath your shoulder and your front foot in front of your left shoulder. Try dialing your stance width back to 19 or even 18 inches. Feel free to change it back after we've learned some new turning movements. With a narrow stance, try riding "Cowboy" (i.e. with your legs bowed out a bit).
Lesson#3 - Stance alignment - bindings
When the upper body is open to the lower body it puts torque on the lower body that induces rotation/instability when the board is flat. Having a larger front foot stance angle reduces that torque but also reduces your ability to power from edge to edge. Try changing your stance to +15/-9 and again change it back later if you like.
Lesson#4 - Stance alignment - binding highbacks
Have you ever tried adjusting your highbacks? Crank them up a notch. If your legs don't get sore, crank them up another notch.
Lesson#5 - Stance alignment - upperbody
Concurrent with lesson 3 ride with your chin touching your front shoulder to see where you're going instead of turning your upper body. Ride with your hands like you're driving a car - both hands on the steering wheel. If you catch yourself with your front hand over the heel edge like pic4 then hit yourself with a cattle prod.
Lesson #6 - Bend your legs more!
Stand in your old snowboard stance and look down. Can you see your toes or are your knees covering them? If you can see your toes, bend your knees more until they are "covered". That should be your "home base" stance. Move your knees 2 inches out past your toes to go onto toe edge. Uncover the toes to go onto heel edge. We need at least that much movement to go from edge to edge. Try this all again in the new recommended stance and see if you can tell a difference.
Now I want even more! Ride with your hands touching the outsides of your knees all the time except for edge change. When you want to change edges, stand up taller and touch your shoulders while you change edges, then go right back to holding your knees for the rest of the turn. This should be easy, but make sure you do it with leg movement instead of hip movement. Repeat with hands touching where the tops of the boots meet the legs, then reaching for the shoulders for edge change. Easy? Try again with hands touching the toes. Easy? Try again with your hands touching the board on toe sides and grabbing the edge on heel sides. I'm getting too old for that one and most people don't need to get this extreme. The point is you should feel the turns getting easier as you get more vertical movement going on. The more vertical movement you have, the easier it is to pivot the board underneath the feet versus pivoting the board by kicking the back foot out. The faster/steeper/bumpier we ride, the more movement we need in those legs baby!
Lesson#6 - Knee pointing
This isn't the way to ride. It's a way to ride. With your legs bent more, try pointing your knees into the new turn instead of pointing your shoulders.
Lesson#7 - Larger turns
Try making some turns where you finish the turn going uphill. Your new turn should engage the downhill edge of the board to get it started! You're going to have to keep your weight more centered. Pic3 is better than pic 4, but you could still get a bit more forward than pic3. You're also going to have to stop pivoting that back foot out for turn finishes. Let the back foot follow more. Let the front foot handle more of the turning load. Do you ever feel your right leg get more tired than the left? Just sayin'.
I'm not trying to "fix" anything here. I'm trying to give you more options that should help you carve better if you want to carve, ride faster if you want to ride faster, feel more comfortable on steeper terrain when you go there and so on. Can you get all this done before the snow melts?