The only factual argument that matters is that ZEFs are inside someone else's body while infants are not. The change from the first state to the second is significant for that reason. Abstract notions like level of development or chances of survival are grey areas; whether someone is inside another person or not is black and white. If this doesn't make sense, imagine that you're standing in a crowded bus, and the guy next to you has his hand in your pocket, or in his own.
What you're saying is the pregnant person's suffering in being forced to continue a pregnancy she doesn't want (and it is suffering if you've ever known a pregnant person) means less than the life of the ZEF. So a ten year old rape victim is required to risk her mental, physical, and emotional health, and even her life because the ZEF's life is more important. After all, she only has a risk of death; the aborted ZEF is definitely going to die.
My question is whether you would apply the same equation to yourself. Let's say an insane person is going to break your arm, and the only way you can stop him is to kill him. According to your calculus, you have to let him break your arm, because his life outweighs any pain you would experience. In fact, if he was going to break your five year old child's arm, you must let him if you can't stop him without killing him.
So maybe death doesn't always outweigh suffering. I hope we can agree that forcing a pregnant person to continue an unwanted pregnancy would cause them great suffering, while aborting a ZEF that's unable to feel pain causes negligible suffering. A moral position seeks to minimize suffering wherever possible. The opposite is sadism.
Nobody has an abortion in the third trimester because they couldn't make up their mind earlier. These only comprise one-half of 1% of all abortions anyway, and amount to major surgery. These are all tragic situations where the baby was very much wanted, but either the mother's health wouldn't permit her to complete the pregnancy, or the ZEF was found to have a genetic anomaly that would have sentenced it to a short life of inconceivable suffering.
Ironically, outlawing abortion increases the number of late-term abortions, as women who otherwise would have had their abortion earlier must now either save up to travel somewhere where abortion is legal, or find a clandestine provider.