What materials can be used to stiffen a leather bag?
One thing not on your list is steel wire.

This is a sort of steel framing I suppose. But it can do a lot while remaining light and flexible. I would think you want something in spring temper, so perhaps piano wire of sufficient gauge. I suppose whenever I've seen these it's been mild steel though, somewhere around .050-.100" diameter.

"U"-shaped members on each side and each end might do the trick. They could be sewn into seams or otherwise concealed.

Interestingly, these members resist torsion (twisting) as well as bending. So, for example, if you wanted a bag that didn't flop down but was free to pull apart for access at the top (and pull together at the top to close), you could place a square wire frame at the bottom and a U or square frame on the sides only (not the ends). The frame would tend to make each side "hinge" in and out as one, roughly like the cover of a book, rather than having to pull each end separately to open.