The information is out there if you know where to look. This is good place to start.
Most of which mostly pertain to current members, or appointees since the start of the game. There's little to no information regarding Council members in the early Cold War or during the Great Galactic War. And considering people like Vowrawn and Marr are considered anomalies with their years long, or even decades long tenure, one can't assume who held what seat more than a few years before the start of the game.
Regardless, I for one would like to know which Sphere was Darth Soverus heading. Maybe Military Strategy?
Most likely, considering Defense and Offense are spoken for.
Always rubs me the wrong way to see new people appointed to the Dark Council and the Sith Inquisitor not partaking in it. I do understand the complexity of creating SPECIFIC NEW for every single time a fellow councilor is killed but it makes the character's role feel fairly underwhelming.
For some, I could see it explained away as the 'Military' spheres wanting to keep appointment decisions in-house. And with Marr taking charge of things, it's likely all one needs is his approval to gain a seat. With Ancient Knowledge being a rather insular sphere, I can understand the other Councilors feeling like they don't need to acknowledge or include the player in every decision. As such, the player is granted the most leniency, but is also kept out of the political realm.
It'd be good if in the future, they start to acknowledge that the Inquisitor is no longer the fledgling member of the Council, with new appointees looking up to them the same way they would a senior member. Maybe get the chance to swat a fellow Councilor down if disrespected.