I haven't used that many, but I've never used a photo backpack that I found exceptional. If you want a backpack, I'd seriously consider the suggestion others have made about using a backpack designed for hiking and using something inside it to protect your gear. It sounds like the camera/lens is the only thing you're carrying that really requires any protection.
There's no such thing as completely waterproof. If "waterproof" is important, I think you do need to consider something with a separate rain cover (or get a generic rain cover). Even those always let some water run down the side of the pack adjacent to your back. Most of the photo backpacks I've looked at with rain covers also have the straps attached at the top of the bag. This prevents the rain cover from wrapping around the bag as much as I'd like.
You are not carrying too much of a load. Have you considered using a good belt system instead of a backpack? I use one all the time, although I haven't been hiking with it. The stuff I use is from Kinesis (www.kgear.com
) but Lowepro and Thinktank also make systems. The Kinesis stuff is designed by someone who wanted gear for trails and backcountry. Thinktank seems to be geared more to commercial photographers and photojournalists.
I switched from using a backpack to the belt because I find it's more comfortable to have the weight on my hips (where it should be with a good backpack as well but rarely is with a photo backpack) and it's a lot cooler (temperature-wise) than having a pack against your back. This works well for me in spring, summer, and fall. In the winter, I usually end up using a backpack.
The belt also makes it a lot easier to get to your gear. I typically have pouches all the way around my waist. In the back I have lenses or water. It's very easy to reach around to access those. The camera with a lens is in a holster-type pouch on my right side. Accessories, smaller lenses, a second body, etc. are on my left.
At a minimum, I'm typically carrying my 7D w/ 70-200/2.8, a 300/4, a 10-22, a teleconverter or two, maybe a second body (SL1 or EOS-M), and various things like a phone, sunscreen, etc. I spend a lot of my time shooting where water is readily available to I don't always carry some. I do have a pouch that holds a 1L Nalgene (or similar) bottle that fits on the belt (and sometimes replaces one of the lenses).
In light rain, I do not worry about waterproof-ness although some water does get through some of the seams and zippers. I have a rain cover that wraps around the entire belt and all pouches, but I will admit that it can be a bit of a pain to put on and get off (too much stuff on my belt) and it makes it difficult if I am actually trying to shoot and not just protect my gear.