Fairphone 6 is switching to a new design that's even more sustainable

ahhh I wish GrapheneOS was supported on these!

CalyxOS support Fairphones. It is a better option anyway, also supports bootloader relocking etc.

In which ways is CalyxOS "better"?

This comparison is pretty damning: https://eylenburg.github.io/android_comparison.htm

Calyx is _not_ a hardened OS, and runs on devices with insecure hardware and firmware (like a Fairphone). Additionally, app compatibility suffers because they use microG instead of proper sandboxed Gapps, and also lacks many QoL features that guard against hostile apps (storage scopes, contact scopes, ...).

Why would you call the Fairphone an insecure device? It has long update support and can re-lock the bootloader, which is like the one criteria ROMs like that pick as the security feature. What is your attack scenario, what your security criteria here?

microG is not a drawback, it's a proper FOSS implementation, which I vastly prefer to running Gapps in a sandbox. App compatibility has been perfect for me.

The main criteria for it to be strictly better is that you do not give root to a dev that hallucinates enemies and then send their goons to attack them. See https://www.youtube.com/watch?v=4To-F6W1NT0. It's nice that Graphene has a hardened kernel, that helps nothing if you can't trust the developer - different attack scenarios.