I had this same long conversation with Tim once. Basically he steered me to judo, which will have what you're looking for, shoebox: Takedowns/anti-takedowns, groundwork. Compatible with and will shed light on, your taijiquan, and vice versa.
After you do that, eventually you will probably realize that "self defense" has none of the assumptions you make in your line of questioning. cloudz' comment about his daughter gets more to the point. You may be 90 and then realize that "self defense" will not really be about your anti-submissions or anti-takedown skill, even if you are ex-SEAL Team 6, pro MMA, multiple blackbelt, Chuck Norris, etc... But you should still go through this path. It is fun, intellectually stimulating, good exercise, good socializing, self-confidence-building, makes you an educated sport MA fan, etc., etc.