I've tried a few bags in my time, and I was a fan of LowePro's Slingshot bags, until these starting to become too much to carry on one shoulder. Even with the belt strap, my last 300 was too much. Partly because my kit evolved, but also because of me not getting any younger...
Still "only" in my 40ies though.
I've settled on the Manfrotto (ex-Kata) Pro-Light 3N1-35 now. This can sling as required for fast access (both sides) and can be converted to a proper backpack as well. Any straps not in use can be tucked away. On a flight I often tuck away *all* straps when I put the bag in the overhead compartment, just to avoid the straps catching onto another bag that someone might take out before I get to my bag...
The side access to the main compartment opens in one move and can be securely closed. The inside dividers tend to sag a bit more than with other bags, but that's maybe because I tend to load more because of more space compared to previous bags?
It has ample extra pockets on the outside (for lens caps, batteries, cards), and a roomy top compartment for various stuff. On the back it also fits a large laptop if needed, though I mostly put travel papers in there.
For all weather is comes with a rain cover that has a black (water repellent) and a reflective side (for use under a baking sun).
The tripod attachment is well thought out and can cope with large tripods. But I don't like it too much (it's a separate attachment), and prefer to carry my tripod in a separate tripod bag.
Not saying it's any better than alternative, but you might have a look at this bag as well...
hth, Wim