While I understand your desire to have a small backpack and distribute the weight evenly, for light loads I find a well-designed sling bag is much easier to get into and out of on the go, and no less stable or secure on your back. The term "well-designed" is the hard part ... I went through several sling-bags and returned them or sold them pretty quickly.
After much disappointment like that, I decided to try the Peak Design Everyday Sling 10L and 5L bags because a regular backpack wasn't working for me and a shoulder bag didn't fit the need I have (bicycling and/or motorcycling, with quick access to the contents of the bag). I need both because there are times when I have only a little bit of stuff to carry and the 10L size bag is an awful lot of bag to have flopping about when you only need a little bit of space.
I haven't used the 10L bag yet, but it's larger than what it sounds like you need anyway.
The 5L bag has surprised me and proven to be very handy and useful. It's ideally sized for a small ML camera and up to three lenses; it works well for a Leica M and two lenses. I can carry it with the following gear:
That's an iPad Pro 9x7, Leica M-D, Summilux 35, Summicron 50, glasses, AirPods, wallet, keys, and iPhone 8 Plus; all up weight is 6 lb 7 oz. A bit more than I usually carry, but useful to explore the capacity and weight.
My more usual carry kit these days is with the Light L16 or Polaroid SX-70, which cuts the bulk down and reduces the weight to about 4.6-5.1 lbs, about the same as with the Oly E-PL7 and two zoom lenses. Drop the iPad pro with any of these three cameras and all up weight goes down to 3.25.
The Everyday Sling 5L proves a very handy bag, the design is right. I've now ridden the bicycle with it for 400-600 miles with the iPad and Light kit, and even a spare jersey and gloves jammed into it in case the weather got overly cool, with no undue effects to my shoulders or back. It doesn't move around if you have it snugged up onto your back properly, and the slide equipped strap lets you open up the strap to a longer length easily to be handy for access when shooting or you need your wallet, etc.
It's worth thinking about.
(I do use a photo backpack, albeit infrequently: A Think Tank Photo Trifecta 10 fits the Leica SL + SL24-90, SL90-250, and Super-Elmar-R 15mm f/3.5 beautifully, along with plenty of stowage for other incidentals. But that's a bulky, 18lb total weight kit; I really hate schlepping it around for very long, no matter how good a bag it goes in!)
G
