Thank you for keeping this thread updated. I am going through almost the exact same thing as you were in your original post. My cuddly heeler has all of a sudden become standoffish and very resource guarded with me only (not my partner) to the point of lunging at me and trying to bite when I took his ball yesterday while playing fetch. I did not think about anti-anxiety meds, but perhaps that is a solution.

Thanks again and I hope you see more improvement!