Re: psrr of bandgap
you could increase the impedance between the supply node and the output node.
psr=v_supply/v_output
v_output=v_supply*(Rout)/(Rout+R)
v_supply/v_output=(Rout+R)/Rout
R=Resistance between supply node and the output node
then for increasing psr, you could increase R,
You may increase R with a cascode as current mirror or
you can use a pre-regulator.
go to chapter 4 in this book for more information, Voltage reference: from diodes to precision high order bandgap circuits; Gabriel A. Rincón Mora