Peter
here are some thoughts, some read, some advised, some personal.

I agree that sternum strap is very helpful. Exact tightness is hard to predict, but experiment.

The shoulder straps really should not take that much weight. One should be able to release them, and have the pack supported by only the waist belt. The shoulder straps just keep things from swaying side to side or back to back. ( An exception might be when your pack is too heavy, or ill-fitted) . There should be daylight between the top of your shoulder and the inner curve of your shoulder strap as it goes over your shoulder. The strap may rub and redden your collarbones as it pulls backwards, but not the top of your shoulder.

Have you tried tightening/loosening the small straps that go between the shoulder straps and the top of your pack? Experiment.

Have you tried external frame pack ( more upright posture) versus internal frame ( more hunched forward) ?

Have you tried varying weight distribution inside the pack? Sometimes the denser bear can weight in the bottom versus the top makes a huge difference in center of gravity, or even leverage on the shoulders fore and aft. Sometimes, the right packing for comfort ends up being counter-intuitive.

Finally - do what I do...relax the trapezius while walking. How? One can do it simply by just altering position. Walk a few minutes with your hands straight up in the air. Yes, it takes muscles, but it breaks up the isotonic tension of at least some of the muscle groups.

Ultimate solution for me: all of the above... AND... walking like a crucifix. I place my walking pole (or usually longer single staff) across the top of my shoulders and across the back of my neck. Then walk with both arms wrapped over the poles. Looks goofy - my companions know it is me from afar. Do it for a minute or two at a time and intermittently whenever you feels tightened up. Again, it just allows use or relaxation of a different muscle groups.

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Harvey