Thanks again. You can manually assign DNS64 servers (Google, Cloudflare, etc) to the interface to avoid address translation performed on your side.
If you need to extract the address only, then you only need to parse +CGPADDR output.
You can avoid using echo as shown here: Get result of AT command to a variable in Bash script - #4 by AndrewZ
Instead of running odhcp6c you can create or update a virtual interface, you can see examples in various OpenWrt connection scripts like qmi.sh etc.