I read elsewhere that vampire spawn can NOT create more spawn, though I'm unfamiliar with the process myself. As I understand it only way for it to work would be to kill Astarion's master turning Astarion into a full vamp then becoming Astarion's spawn and killing Astarion to become a full vamp.
Or become a spawn directly from Astarion's master and kill the master to then become a full vamp along with Astarion.
You're right, even Astarion confirms a spawn can't create another spawn. He also says killing the master isn't necessary to become a full vamp, all spawn has to do is drink his master's blood, but it's very unlikely a vampire lord will allow him to do this. I think Astarion would let MC to do this if he becomes a vampire lord and if their relationship were positive.
