It would make sense that Phobos, like Earth's moon, was the result of an impacting object versus a captured asteroid. I would think that captured objects, such as D-class asteroids would eventually have an orbit degrade over the course of a few millennia.

Well, Phobos is predicted to spiral in within 50 million years or so.

Wouldn’t an impact that large have destabilized Mars’s orbit?

Earth had a bigger impact that result on the creation of the Moon, and we are here.

That’s never been substantiated beyond a theory. The same question would exist for that theory as well.