"Star Wars: The High Republic" is a better starting point if you're interested in delving into the lore surrounding the rise of the Sith. While "Star Wars: KOTOR" (Knights of the Old Republic) offers insights into Sith history, it's set thousands of years before the main cinematic saga.
The High Republic series, which includes novels and comics, explores the Jedi Order at its peak and the threats they face, including the emergence of the Nihil, a group of space pirates, and the potential for Sith involvement. It gives readers a fresh perspective on the Jedi's challenges before the events of the Skywalker saga.
If you're keen on exploring the deeper nuances of the Sith and their philosophies, the "Darth Bane" trilogy or the "Star Wars: Dark Lord" novel might also be interesting. These works dive into the Sith's ideologies, teachings, and the creation of the Rule of Two.
Ultimately, it depends on what aspect of the Sith you're most interested in exploring!