It’s probably difficult for some folks to wrap their heads around Vader being the Dark Lord of the Sith whilst being subservient to a non-Sith Palpatine because since 1999, the Sith have been portrayed as apex predators who have a monopoly on dark side mastery. But this is purely an invention of the prequels.
You do realize that the fact that Vader calls Palpatine his master obviously means Palpatine was a Sith in TESB and ROTJ? Like yeah you can rationalize a different explanation but it’s clear that’s the intention. There’s no need to jump through hoops for another explanation. The scripts for all three call Vader a Dark Lord of the Sith and the Emperor is his master.
“Master” obviously does refer to their relationship as Sith Lords because none of the imperials are calling him master, so it’s not just a term recognizing his authority. Vader is his apprentice and he tries to replace him with Luke, as he calls him his apprentice twice.
There’s literally no reason to not make Palpatine a Sith other then to just be different for no reason.