“Only a master of evil, Darth.” Seems a perfectly fitting place to use his title, personal moment or not.
Anyway, if Lucas was going to change it to make it more “personal,” why would he change it to Anakin?
I think if Ben put more emphasis on the Darth, said it in an vaguely spiteful tone, I’d be fine with it. Almost as if to point out that he’s just another Sith Lord, his own identity has been stripped away from him. He just says Darth very casually, IMO, as if Darth were a first name. I think either Vader or Anakin would’ve worked too.