As previously stated the MX10003 cannot do CGNAT - only inline 1-1 static NAT.
With that amount of CGNAT traffic, the pricing of an integrated CGNAT solution based on MX480/960 with MS-MPCs does not make sense compared to eg. a cluster of SRX4100 or SRX4200 firewalls doing CGNAT.
An MS-MPC is more than twice the list price of a SRX4100 gateway capable of doing 20G of IMIX traffic and hold 5 million concurrent sessions - and you can later via a license upgrade your SRX4100 to SRX4200 if needed providing roughly twice the throughput and scale without new hardware.
...and you can of course still stick with your current CGNAT solution 🙂
I hope you can use this input for your evaluation.