But the Sith Empire had been consolidating their power after the Treaty of Coruscant was signed. They even recently took Taris from the Republic. Shouldn't there a stalemate between the Republic and Sith Empire at this point in the Second Great Galactic War?
No
They are losing they lose Corellia and have to with draw from the Core worlds including Balmorra, the Dread Masters Turn on them and then Regius (the Ilium Grand Moff) goes rouge, so there is no stalemate the Republic is winning at the end of the game.