I've done a lot of work with 4-5 oz and 6-7 oz. Personally, I think either would do well. But I'd probably stick with the 4-5 oz as it will be a tad easier to work with, and should still be plenty strong and stiff for a typical "day" sporran.

One thing you could try is to wet the leather and let it dry. Usually when I wet-mold the leather to shape, after it dries it is as hard as a brick. I then have to oil it profusely to get it somewhat soft again. But if you wet it and let it dry, then simply dye it and finish it, it will likely remain a little more stiff than it otherwise would.

I'm not familiar with waxing the leather as a finish, so I don't know if that's contributing to your issue. But you could try just dyeing it and then finish it with something like Neat-Lac, which won't soften the leather and will actually help stiffen it just a wee bit.