GrapheneOS is great, and it's what I currently use, but it is ultimately a hardened Android fork. One downside of that is it is completely reliant on manufacturer updates to continue to support a phone. Once a manufacturer drops support, the Graphene team must also drop support, as they are reliant on the closed source GPU/hardware drivers that are tied to specific android kernel versions.

PostmarketOS is not based on Android whatsoever, it's a Mobile focused Linux distro using the mainline Linux kernel. It uses open-source drivers for the GPU and hardware which can be maintained and supported for decades, and is completely independent of Google's influence. However, it's still currently rough enough around the edges that it isn't ready as a daily driver, which is why it'd be so helpful for us to donate to it so they can hire more developers to polish it up, as they recently did to improve the audio support of Qualcomm devices.

Graphene is Pixel only and aims for privacy and security.

PMOS aims to bring Linux to the maximum amount of devices (phones, Chromebooks, tablets, QEMU) to give them a life beyond the manufacturer's support.

I would compare PMOS to LineageOS over Graphene.

It's still ultimately downstream of Google code, though

Technically you cannot since they basically develop Android.