Since most (all?) thumb brakes are designed to work in addition to the existing rear master cylinder, I don't see how adding one would do anything to improve your problem. By more than doubling the plumbing, it seems to me that you would only add opportunities for increased softness.
Some members have reported that replacing the rear reservoir solves the problem. For myself, I just bleed the brake when needed, and it doesn't bother me. I might try an aftermarket reservoir, but it's not top of my list. I certainly would not consider a thumb brake; they are useful when turning right on a race course.