What I took out of RoS which I really liked the idea of was how it explained Palpatines “stike me down” lines both in RoS and return of the jedi. I like to think that meant that if Luke struck him down in return of the jedi with anger he would of possessed Luke the same way Rey would of been possessed if she stuck him down in RoS through the sith ritual.
I interpreted the line “I am all sith” as meaning the past sith which had been possessed in this… unnatural way.

I’m sure theres some overlapping story issues via novels\comic\sticker album somewhere that destroys all that but I personally enjoy this idea of the events.