PSR of bandgap
I don't think that higher op amp gain can improve the psr. As I see, the following idea is can improve the psr.
1. Take advantage a pre-regulator inner power for the bandgap.
2. use the cascode current mirror.
3.refine the op amp's design. Set proper DC gain and make sure the compensation is ok. Generally, 60dB gain is ok.If the gain is too high, it is difficult to compense the OP AMP for enough PM.
4.connect a output capacitor can do it.
Depend on your bandgap's structure ,complex and the SPEC parameters, you can choose ideas to improve it PSR.