AOSP is not a usable product and it is full of Google stuff. GrapheneOS and LineageOS (and derivates) make it usable.
GrapheneOS was the first at complete degoogling, others followed, but still have no secure solution for apps needing Google Play. MicroG is proprietary Google code, ran unsandboxed and still unreliable and less secure.
So GrapheneOS really is the only usable OS right now, it is FOSS (even in the BSD way that companies could make their proprietary fork) and other projects should base on it and weaken its security to make it run on existing other phones. This is still better than alternatives, but it is a hell of maintenance as all that security causes infinitely more bugs than just using AOSP (which is rock solid) and adding some apps lol.
Please donate to GrapheneOS, the project is not yet fully sustainable and they are doing incredible work.