Physics is the real fix here. Hanging stuff off the outside moves the center of gravity back, which actually makes the pack pull on your shoulders more, not less. Putting the heavy dry bags inside and close to your spine is what gets the weight down onto your hips, the outside stuff should just be light junk you barely notice.

Ouch, getting doored at 18 mph sounds rough and I'm glad you made it through that. Your buddy isn't totally wrong that the door zone is dangerous, but hugging cars is way worse because you've got zero room to dodge anything, and drivers behind you can see you better when you take the lane anyway. Ride safe out there and trust your gut on this one.

Gosh, that's just how it goes sometimes, the fancy stuff never beats the simple basics you already own.