Really his motivations change once.
He is manipulating both Rey and Ben to see who will come out on top. I don’t think he cares if Rey is his biological grandchild. Whoever kills who, whether it be Ben or Rey, will make for a stronger vessel. Once Ben turns to the light, he focuses on Rey and tells her what she wants to hear, “I never wanted you dead.” That’s what he’s always done, lie and manipulate.
But once Ben and Rey are together, he realizes how strong they are together and decides to revitalize himself. I think he would have preferred a newer, healthier body, but healing his own body will suffice.
But that’s just my take.