Lucas said “balance” was removing the Sith.
It is 90% of the time, and then that other 10% it’s the Mortis arc of TCW where balance is pretty explicitly an equal amount of Light and Dark.
From a narrative perspective, Lucas introduced the Midichlorians and Darth Plagueis in chapter six.
No, he introduced midi-chlorians in chapter four.
AND Plagueis, dude. AND. The Midis may have existed in four, but six was when they became relevant to the narrative. As an example, Stannis was mentioned several times in Thrones book one, but his relevant influence to the current narrative started in book two.
In ROTS, by specifically introducing an entirely new character and his Midi-abilities, Lucas was laying down narrative threads for his microscopic plot in the next chapters.