You are progressing as expected. It is good you have nearly full flexion and that amount of external rotation at neutral is good. What is your range at 90 degrees of abduction?
Once you attain good rotator cuff strength you should then expect the ranges to improve further. I generally tell people that full rehab after shoulder surgery is up to 12 months. The reason you get the damage in a shoulder is because the rotator cuff is weak. Surgery makes the rotator cuff weaker so you do start quite far behind the 8 ball. And by the end of the rehab you will probably have better external rotation strength than before the surgery.