Hey FizzzyCal,
I understand your dilemma completely. I've been training in Tae Kwon Do Moo Duk Kwan since the early 80s. I Love it, it has great Self Defense skills and I wouldn't trade my experience for the world. That being said there is always room for more martial arts for your arsenal of Self Defense. Hap Ki Do is another Korean Martial Art that focuses on Self Defense by use of Joint Locks, Throws, and Strikes (among many other things including weapons, depending on the Kwan). It stands alone as a very powerful and effective Self Defense System. It has been a great experience for me so far and I'm learning a lot that I didn't learn in Tae Kwon Do over the years (because it's a different martial art). I think if you research it thoroughly and find the Style of Hap Ki Do that works for you, it will serve as a great additional Self Defense Art.
My 2 Cents.
Bob