The
Sith Order and
Dark Court are probably your two best bets rn.
The Sith Order deals in big plots and underlying eldritch horror themes, it's more of a cult than an empire, with a church and dominant
religion that seeks to combat the force. it is currently locked behind the
Blackwall, which keeps them isolated and under the emperor's control but also makes them the boogeymen of the galaxy, with information about them outside the blackwall almost nonexistent. They are currently the dominant sith faction, in fact im not aware of any competitors right now. I've participated in some neat stories among the SO, and some of it's worldbuilding is very interesting and quite expansive.
The Dark Court is a new minor faction created by a sith lord exiled from the SO and her co-conspirators. They take a very different approach in that they don't seek major faction status, but rather infiltration of galactic government at large. While not exactly the same, the rp is more akin to Rule of Two era sith scheming in the background without ever being discovered.
Both are viable options and my character is an active member of both factions simultaneously, though obviously keeps it a secret