You have gotten a good result from your pectus excavatum repair, however, I clearly see what bothers you. It is a little difficult to determine the best method to fix this without an exam, but implant placement and perhaps scar revision should be able to improve this problem significantly and give you better breast definition.
Fat grafting is not a solution for symmastia. Symmastia can be a very difficult problem to correct because access to sew the sternal skin down is so difficult. You have the advantage of having sternal scars from your pectus repair to do a more effective symmastia repair.
If you want to be much bigger ,then implants will help. If you want to correct the depression and asymmetry, then fat grafting be an excellent option in hands of surgeons that good at fat grafting.
Synmastia is a problem that is caused by separation of the superficial tissues from the deeper sternal connective tissues. Synmastia is often referred to as "Uni-boob". There is absolutely no reason to perform fat grafting as this is not a problem of insufficient soft tissue deposits so that answer puzzles me? Correction of synmastia is a surgical problem and the correction can be effected through your already present sternal scar. I strongly suggest that you find an experienced and competent ABPS certified Plastic Surgeon to repair your synmastia. Good luck!