Just eyeballing the recipe, Colin, and both the Bake Mix and the 'main' recipe are topheavy on soy, both flour and isolate. This could definitely be contributing to brickdom.

The components of the bake mix are already in the main recipe and the soy needs to be cut down anyway... ditch the 'bake mix' portion, double the flaxseed meal, halve the wheat bran, ensure the protein powder is a whey powder, use a full Tablespoon of baking soda/powder, and change out the club soda and sour cream to a Cup of buttermilk or more, enough to moisten the batter to a 'muffin batter' consistency.... see what happens (ok, so I'll see what happens because that's where I'd start). If you have other sweeteners, combine to get the 1-Cup sweeteness equivalent rather than use all Splenda.

Oy, and add some nice seasalt, maybe a half-teaspoon?