Five Women are physically average in every way. Medium height, medium weight, medium pretty. With nothing to distinguish them they fade into the background.
For Sarah Connor, that’s the point.
You make Sarah an Enneagram Five because you want the audience and the characters to see no reason this person should be targeted for termination. It’s brilliant. Why her?
A Five is also someone who would believe a wildly improbable tale. A cyborg from the future? Pff. Because of their observational skills, Fives would put the clues together and accept the preposterous, and even change their life to suit the new reality.
In real life Five Women are often the teachers and church volunteers and PTA mothers. They quietly run society. When Sarah accepts the mantle of Mother of the Future Hero, she’s only fulfilling this Five-ish role.