CalyxOS relocks the bootloader and they supported the FP5 right after launch.

CalyxOS is not a hardened OS, and GrapheneOS requires more than than just relocking the bootloader.

Fairphone’s devices do not meet basic security requirements for hardware, firmware and the software device support including drivers. Please look at the hardware requirements at https://grapheneos.org/faq#future-devices and check for yourself how many of those are provided by the Fairphone. Even the Fairphone 5 has a CPU core from 2021 without even PAC and BTI.

Ref: https://discuss.grapheneos.org/d/7208-8y-security-updates-on-fairphone-5-will-the-devs-consider-porting-grapheneos