I'm really trying to make a great low fat muffin. No matter how hard I have tried with this one (I am not over stirring), I can't get these to rise past a mini muffin height, and this batter is so sticky, it's all I can do to get it off the spoon. Any insight/suggestions? https://www.averiecooks.com/skinny-blueberry-muffins/
Muffin mix should be a pourable batter, not a thick dough, your mix is too dry. If it's too dry and claggy it won't be free to expand, and you don't get a rise from water vapor. Try adding more cashew milk until you get a pourable batter, and try it again. You want it to be ribbony, not so loose it evens out right away.
Try a different sour milk product.
I am suspicious of the pectin and guar gum and other stuff that turns up in Greek yogurt. Suspicious for baking and cooking purposes; I think that stuff is fine to eat as yogurt. Different "greek yogurts" vary a lot as regards what thickeners are in them besides milk. Maybe the author used something different than what you have.
I never bake or cook with nonfat yogurt but if that is an important part of this, you could try a brand with minimal additives. Or if your goal is to make good muffins you could use plain full fat yogurt. Or substitute sour cream thinned to the correct consistency with some extra cashew milk. Sour cream makes good muffins.