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.
EDIT: And yeah, I think one of the reasons the original plot was changed was to give Kylo more agency. If Palpatine wants Rey dead, and he can clearly be against Palpatine.