When I started my recovery journey my sternum had very little flexibility. I could push it out the way you describes arond 5 mm. I ask myself; have you made high precision measurements of before and after you started? One thing that I also am woundering about; do you sit down a lot during an ordinary day or do you have a job that makes you physically active?
If the specialist has told you that your flexibility is very limitid I can only recommend you to do incline pushups, that helped me very much in the beginning to loosen up the sternum. It is simply because this exercise streches the whole thorax. I hope this will help.
My method has been proven to work for some people but beacause this is not tested clinically in a big study, it is impossible to say that it works for everyone. But I Think that it may do so even for more hardened sternums, one just have to be very motivated and stricly being active. One of the worst things to do I belive, is to sit down for very long periods during a day. Even for me, the sternum sinks back if I do that.
Hope this will help you in some way
Björn Å.