I've tried several large lumbar packs with shoulder straps and have come to the conclusion that the only advantage is that they are a bit cooler. Most are too complex with as many or more straps than a backpack and don't save much in the way of weight or expense.
With back issues, I think you get more from a lightly framed pack with good weight transfer to a hip belt.
For day hiking setups, I have considered using a small light pack like many of the "stuff sack with shoulder straps" models. The problem with them is being uncomfortable with anything more than a fleece and a snack, and water being the heaviest item, along with a pound of essentials, camera, cell phone and other hardware".
To relate this to the half pack, I have looked at adding a hydration-oriented lumbar pack to the small backpack. That combination is usually a few ounces heavier than something like a simple rucksack with water bottle pockets, but it does get all the heavy items on the hips and leaves just the light puffy stuff in the backpack with less density and weight. It also allows easy flexible movement in the lower back, a bit more ventilation, and you end up with two handy-dandy packs.
What I haven't seen is a good lumbar pack made from SUL materials, good stabilization, and a couple water bottle pockets. The bruisers like Mountainsmith are totally overbuilt, while the UL running-oriented models lack storage.
I would spec a Cuben pack with a roll top that is 4-5 liters volume, a water bottle pocket on each side, a foam reinforced hip belt and a zig-zag strap and buckle setup to get some stabilization. Team that up with an uber minimal backpack like a Zpack Zero. Your water and heavy hardware goes in the waist pack, with clothing in the backpack. I think that is simple and versatile.