I know that the title “Balance of the Force” has been used on some Star Wars content in the past, but the title “The Last Jedi” was also used before in the past too.
Wasn’t the first time either:
As for Episode IX’s marketing, I think that it will be marketed as the last installment of the sequel trilogy, but not the main saga.
It already is being marketed as the last of the saga. This is from the original cast announcement for IX:
STAR WARS: EPISODE IX CAST ANNOUNCED
Star Wars: Episode IX will begin filming at London’s Pinewood Studios on August 1, 2018. J.J. Abrams returns to direct the final installment of the Skywalker saga.