It is true that both Augustine and Aquinas wrote of a delayed ensoulment, but I don’t believe that either declared it legitimate to destroy the unensouled, but still animate, embryo.
Great theologians sometimes speculate, and not all speculation rises to the teaching of the church.
St. Luke seems to have thought the soul entered the embryo at or very soon after conception—he has St. Elizabeth speak to Mary concerning “the fruit of your womb” and call her “the mother of my Lord” (1.42-43), at a time that may have been only a few days after the Annunciation.