If not a clump of cells, what is a fetus? Science and biology don't help you here. The problem is that the SDak lege, faced with an inability to pass a binding law that makes a fetus an independent legal person, sought to end run this through a dubious invocation of biology for precisely the reasons you identify:

It's a situation where the analysis (and factual honesty) are shaped to reach the intended result

Your personal view of what is not a "separate entity" is getting in the way of reasoned analysis.

BTW, you do know that an old lady on lifesupport can have her plug pulled ismply for the inability to pay her bill right?

So if a hospital has the legal right to terminate a life simply because of an unpaid bill, why does a fetus get more rights to demand service from a woman?