Then we're arguing about definition. I consider "personhood" as what makes a human being, you apparently believe mere biology does. I can make a case for protecting the life of a person, I have seen no compelling argument for regarding a mere biologically human organism as anything worthy of particularly special consideration. If you'd like to make such a case, I'm interested to hear it?
I think you gave a pretty good summary of our differences.
I won't make personhood a criteria, because it is difficult to define, and impossible to pin down when it arrives. IOW it is worthless as a criteria. You might as well have "when we get a soul" as a criteria.
On the other hand, we know what human means (as opposed to chipmunk or worm or redwood tree) and we know what a complete human organism is, as opposed to simply a part. Finally we know when a human organism is alive. And so, we can identify human life, from it's conception to its death, from its nacient state til its deterioration in old age.
If we must have dividing lines, they must err on the side of caution. For example, if we are going to grant rights to fetuses that can live outside the womb, we must grant rights to some fetuses that won't live outside of the womb, lest we accidentally kill some who would.
If we are going to have a crime called infanticide, it makes no sense to have conflicting stages of development. For example, we can't say that an eight month old baby that survived birth but is smothered is a victim of murder, but a nine month old fetus that is aborted just before labor ensues is not murder even though it is much more developed.