I think the “I am all the Sith” line could still work in the context of Palpatine presumably carrying the knowledge of dozens of generations of Sith before him. It doesn’t necessarily have to mean he’s all the Sith in one body.
I rather have that be the rational than what the implication is from the movie … it didn’t jive at all with the previous movies. As long as the other lines are removed it keeps that just at ambiguous as Rey saying she is all the jedi … which she clearly isn’t.