GrapheneOS and CalyxOS are much different. GrapheneOS is a hardened OS with substantial privacy and security improvements:

grapheneos.org/features

CalyxOS is not a hardened OS. It substantially reduces security. It regularly goes months not shipping critical security patches.

CalyxOS is closely related to LineageOS and heavily based on it. LineageOS is not at all security-focused but rather is focused on broad device support which comes with major compromises. It adds a lot of attack surface and it doesn't keep up with major releases.

Compatibility with Android apps on GrapheneOS is also much different. GrapheneOS provides our sandboxed Google Play compatibility layer:

grapheneos.org/usage#sandboxe…

Can run the vast majority of Play Store apps on GrapheneOS, but not CalyxOS with the problematic microG approach.