"unlockable bootloader" is the requirement to flash anyone alternative on the phone, yes, but the GrapheneOS you mentioned will support _any_ device that is "flashable" and secure enough: https://grapheneos.org/faq#future-devices

Currently it's only Pixels from 8 up.

Other alternative firmware projects don't seem to be too concerned about security (eg they don't support relocking bootloader, don't support secure boot, don't release patches for months), so they're not really in the same ballpark ALTHOUGH I agree that they still might be better option than stock OS on the device abandoned by the vendor.