“You’re just a phantom, in a failed clone.”
“You’re just a ghost, in a rotting shell.”
I feel like some variation of that has a nice ring to it but gets the idea across. He is a failed experiment, clinging to a life that has long passed.

I liked the idea of what Sasquatch said, if his last words were just “Rey” or something wouldn’t feel too on the nose. And I think the moment when they sense each other, right before she passes the saber to him, works better with no dialogue.