What you really need are stays or a really stiff back panel (plastic?) which will transfer the weight of the pack from your shoulders to the hip belt. Load lifters primarily take the pressure off the tops of your shoulders. Unfortunately, few manufacturers build a support system into their daypacks.
I need the load lifters because the tops of my shoulders are unusually pressure-sensitive. And I have the same issues you do, including lower back issues (which is why I need all the weight on my pelvic girdle, not my spine). Even with a 10-12 lb pack, I still need the support. Part of the aging process, I guess. Unfortunately, most manufacturers don't think that at least some of us need support in a day pack.
I went looking for a day pack a few years ago. In addition to needing the above features, I didn't want the day pack to weigh any more than my multi-day backpack, which is 29 oz. I also wanted the capacity (liters) in the upper 20s or very low 30s. No luck on that one! I still haven't found anything suitable, I did find out that my hips and Osprey hip belts are quite incompatible, and that if I want a woman specific daypack it will weigh almost twice as much as my big backpack. If I could just find my backpack (discontinued in 2006) in a smaller size. . . . In the meantime, at least for fall/winter/spring day hiking, I use my 40L backpack and pull the compression straps tight. It looks ridiculous, but I haven't been able to find anything better.
Edited by OregonMouse (02/24/17 05:55 PM)
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey