Maybe make a backup and treat the new SSD as a fresh install?
Check:
- https://discourse.nixos.org/t/migrating-nixos-installation-to-larger-ssd-without-reinstalling/55740
- https://www.reddit.com/r/NixOS/comments/17e3j5f/problems_in_moving_nixos_to_new_drive/
- https://discourse.nixos.org/t/how-to-approach-moving-nixos-to-a-new-disk/33542
- https://www.reddit.com/r/NixOS/comments/re239d/install_running_nixos_to_another_drive/