That format of bag is nice in that you have everything within view and within reach when you open it up, but yes, they start heavy. Do you think you could live with separate cases within a pack? See if any of the myriad inserts or icu's or whatever each company calls them (Tenba byob series) works for the camera, then find another for lenses and so on, then get a lightweight carryon sized hiking pack and slide those things in.
The downside of this method is the "yard sale" that occurs at your tripod legs.
Panel loading packs (your pack is one) are becoming harder and harder to find unless you buy used/vintage.
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www.511tactical.com
I thought this thing was going to be a winner for me. I found it at a local store, whole front flap opens up, and my Canham 8x10 fit inside. It turns out that while my 8x10 kit did fit, it was uncomfortable all loaded up, and you have to engineer your own padding/dividers since it is not a photo pack. The construction felt fine, and the pack wasn't too heavy. I think it may be a discontinued model.