I'm going to make some predictions and observations:
The "Sith Master" is using a full blown electronic voice changer, rather than just speaking in a DEAP VOICE with maybe some digital enhancement (Kylo). This leads my to believe it's a character we have already met. This would also conform to mystery tropes. My top suspects, in order of probability, are:
1) Master Indara. Faking your own death is a common way for a villain to give themselves freedom to act. It's easy enough to disguise gender.
2) Vernestra Rwoh. Doesn't really have an alibi, but I suspect it would be an unpopular choice with fans of the High Republic novels.
3) Qimir. Pretending to be an insignificant background character is standard Sith playbook. His advice seems a bit too on point for someone who is just a fixer.
4) Sol. He is clearly lying, and has issues with Jedi customs, but appears to be alibied.
I also have a theory about the "Sith master": they are fake. It's pretty much a pantomime Sith outfit. If this theory is correct, someone wants to shake the Jedi out of their complacency by faking the return of their ancient enemy (meanwhile the real Sith continue to hide in the shadows).
A couple of observations/Easter eggs:
CorpSec. The Corporate Sector was the setting for Brian Daley's excellent Han Solo Adventures (now Legends).
Yord has a yellow Sentinel lightsabre (KotOR).
There is a Selkath (KotOR) in the background of one of the shots.