I see Dave Liepmann's already touched on what I want to say, which is that thinking about shoulders is a cart-before-the-horse mentality, and instead you should focus on your use of legs and how they're accelerating your hips, then how the torso drags the shoulders after the hips. Once you've mastered that to the point where you can pass a sizable shock/snap of power up through your torso you'll see from a little experimentation on a heavy bag how much shoulder movement you need to communicate the power into a target - it's not that much.
One of the most important things for getting great reverse punching power is the use of the back leg and foot... I like to keep the foot and knee facing only slightly outside of forwards, so as the thigh tenses it's very directly pushing the hip forwards. Start with the punching side's knee and chain things up through your body. From the knee, the hips, shoulders and arm all engage but only once the level below has dragged them into it. Practice by moving them as late as possible - the hip should be dragged forwards by the knee then thrust by the thigh, the shoulders should lag behind the hips in angle of rotation then be pulled into line by a sudden tensing of the core/abs, the line from elbow to fist should be kept close to pointing at the target, with the fist just forwards of the shoulder until quite late in the punch - don't lead with it.
One thing I like to do is practice "hitting" the bag with my hip, then with my shoulder, then my palm without even straightening the arm to press the hand forwards from the shoulder, then with my elbow, arm-extended palm and fist. This helps you get a "punch" like intensity to the contribution of each layer in your body - then the accumulated power from the entire power chain comes in to play habitually.
While it's not your style, I think there's a lot for most-anyone to learn about the mechanics of good punching from watching Kagawa-sensei explaining (albeit in Japanese) the mechanics of Shotokan - see https://www.youtube.com/watch?v=tdIJVDKJ_Os
Once you've got a great punching action you won't need too much rotation, but you'll naturally find that if you're covering ground to reach the opponent then that's an opportunity to step in a way that helps engage your hips behind your technique. If the opponent sees the preparation and anticipates your punch, vary the height, use an arm bar (if in a real fight or sports rules allowing) or inward knife-hand or reverse-knife-hand / ridge-hand, elbow, kick etc to keep them guessing.