In "Star Wars Jedi: Fallen Order," the storyline does not allow for Cal Kestis to become a Sith, regardless of the choices made during the Journey Plus mode or any other playthrough. The game focuses on Cal’s journey as a Jedi and his struggle against the Empire and the Inquisitors.
While Journey Plus lets you replay the game with all your unlocked abilities and equipment, it does not alter the core narrative or allow for a storyline where Cal embraces the dark side to become a Sith. The game primarily emphasizes themes of hope, redemption, and the fight against oppression, rather than the exploration of a Sith path for the protagonist.
If you're looking for alternate narrative experiences within the "Star Wars" universe, there are other games and media that explore characters becoming Sith or the dark side's influence.