It’s almost like before his “death” his identity was pretty contained to the man “Sheev Palpatine”, but after his “death”, the collective consciousness speaks through the shell of Palpatine.
This was how I assumed things to be on my first viewing in the theatre; that it was Palpatine’s actual corpse being reanimated by some kind of amalgamated essence of evil. Dooku and Bane have canon voices that could be imitated (as do Vader and Maul though they arguably weren’t Sith when they died). I like this idea a lot since it adds a greater finality to the conflict rather than being just Palpatine again.