current mirror psrr
As someone who is currently designing my 4th low power bandgap reference, the best ways to improve your PSRR are increasing the lengths of your transistors so that the PS fluctuations don't have as much weighting, or to bootstrap the PS as mentioned.
If you want to make it much easier to reject the PS, generate the PTAT with a chopping op-amp or an opamp with no (very low) offset and bootstrap the bandgap. This way you will have a low voltage bandgap with good PSRR.