Hi,
Try with ideal OTA to identify that how much is the achievable maximum PSRR (DC 50dB you need or at a certain frequency? over PVT corners you need it?), maybe your bandgap architecture is not capable for this.
Please share your testbench too, maybe it is only a simulation method issue.
And finally, please share OTA schematic with component sizes, annotation results and how you tried to increase gain, these are not obvious. To find a solution we have to see all.