Speaking as a highly experienced backpacker, with typical loads (10-20% body weight), the point of having weight distributed along your lower and upper back is to conform to the spine and somewhat match our natural weight distribution. Putting weight higher is awful, and putting it lower —- while it lowers your center of gravity — is awful for actually moving down the trail. Even day hike fanny packs with a few liters of water in them are annoying enough that I very often just take my regular pack on day hikes vs putting weight directly around my waist. The issue, just in case it isn’t clear, is that as you move back and forth you’re moving the weight with you. It’s much better for it to be held relatively stable against your back.

Finally, I’m trying to work out what you do that you run with 30kg on your back. Trail runners carry significantly less than I do, and even people trying to set through-hike records and who therefore are trail running things like the entire PCT don’t carry anywhere near that weight. Are you in the military? Honestly running with 30kg is pretty damn hard on your body. My knees hurt just imagining it.

Keeping weight at shoulder height is substantially easier on smooth terrain. It’s a biomechanical fact that I also didn’t believe at first. The best way to illustrate this is to think about balancing a stick on end, its significantly easier to balance a yardstick than a ruler, likewise, it is much more difficult for your body to balance weight at your hips than at your neck because you get the advantage of leverage. The higher the weight is, the easier it is to stay “underneath” it.

That said, you have more experience than me: I don't regularly run with 30kg nor walk with 45kg. I'll try your way one day.