I do think Palpatine was intended to be a Sith, though. Once the Sith and the concept of Sith Lords existed, I’m fairly certain that Vader and Palpatine were integrated into it.
Little doubt this is what Lucas intended, but it’s a factoid he apparently kept close to his chest, because Palpatine wasn’t characterized as a Sith in the pre-1999 EU. Darth Vader was characterized as THE Dark Lord of the Sith during the time of the Empire, and as per Tales of the Jedi, only the reigning Sith lord held the “Dark Lord” title. And Empire’s End excluded Palpatine from the Sith hierarchy.
The Sith spirits were gracious enough to make Palpatine an honorary Dark Lord in lieu of Vader, but that Vader had a throne set up for him in their mausoleum while Palpatine didn’t speaks volumes.