Did you ever check what's actually in those fancy bags though? A lot of them are mostly peat moss with a little perlite sprinkled in, so yeah, they can hold way more water than you'd expect. Your 50/50 mix sounds solid for Tucson, but someone in a humid climate might need even more grit than that.

Wait, isn't checking the sender first just basic stuff?

A whole weekend for a 6 inch gap sounds like you went way overboard when you could've just wedged a couple bricks or a paver in there and been done in ten minutes. Slope or no slope, concrete footings are overkill for keeping a dog in the yard.