Of the main ROMs I hear about in the privacy space, it’s Graphene first, then CalyxOS. Divest OS is one I’ve learned of recently that barely has any coverage. Iode is talked about slightly more, in that I think I’ve seen like 2 videos on it. Anything that’s not Graphene is likely based on Lineage. Copperhead is what GrapheneOS started as and I don’t think folks are seriously recommending it anymore. Really, the Android ROM world is pretty small.
To your point about having all of these projects work together, that would be nice and you do see that to an extent. However, folks disagree and so they take things in different directions. In the case of GrapheneOS and CalyxOS an attempt was made in the past but it didn’t work out.