I don’t remember which animated show it was, but I remember him calling some other Darth “Darth” as well
He’s only met Maul, Dooku, and Vader and he calls them by their names.
Although I don’t think it’s necessary, seeing Guiness’ “Anakin” from ROTJ dubbed in place over the “Darth” would be an interesting excercise for sure. The mouth would have to be animated a little extra
That would ruin the twist in TESB entirely.
There is no ‘twist’ in ESB. It’s no longer 1980 and there’s children just born that already know. Besides episode III is set first so anybody watching will already know who Vader is.