I didn't see it specifically mentioned, but you need to know that bikes generally have suspension not to make them ride a lot softer, but to keep the wheels in better contact with the ground, esp. when that ground is really rough. The first dual suspension mountain bikes were specifically engineered to keep one in control while doing bonzai 60 mph charges down rough mountain trails. I have a suspended mountain bike, but I bought it for high-speed commuting on rugged dirt trails: It ain't exactly cushy.
Sure, there have been other designs that claim to be made just for comfort, but the crowd still driving most of the engineering are extreme sports types. You'll find suspensions on a lot of folding bikes, but that's because those bikes have really small wheels, which would be nearly unridable without some mechanical damping. The softest ride you're going to find (probably a fat-tired beach cruiser) will still be relatively harsh to anything rigidly attached as cargo.
Backpacks work pretty well for protecting equipment (assuming you don't fall off), but they usually are not entirely comfortable to carry for very long. If it will be hot, they tend to trap sweat on your back. I like the looks of that Ortlieb case a lot, but I suspect that could still get unwieldy on really technical trails, depending on how burly you are of course.