I know there are extenders available, but I can't say much about them. I believe Steinberger made them for their headless bass guitars. There may be other ways to do the same thing. What I find works best with a 'Bird is a long strap, with the body slung low and the neck pointed up. You can see some of the angle in my avatar, although I'm playing an EB-2 in that picture and the neck angle is actually less than I generally use with a Thunderbird. With the neck angle up, you shorten the moment arm and reduce the torque that creates dive. Also, I use a wide strap. With the body fairly low and the neck pointed up, you can also rest your right forearm on the lower bout, which completely eliminates any need to support the neck with your left hand. (Reverse this if you are a lefty.) It's all about experimenting. Thunderbird ergos are not better or worse than other ergos, in my experience, but they are different. They work perfectly for me and made sense right away. For others, it can take a bit of trial and error. No shame in that.
Some 'Birds dive worse than others. I find that mine generally stay put right were the are, whether I'm touching them with hands and arms or not. I even put them out horizontal and they stay put like that, too. Either I got lucky or you got unlucky, but balanced 'Birds are out there. The main thing now is to figure out how to make sure yours doesn't dive any more than you can tolerate if it's one that is top heavy.