“Sith Eternal” is a name I can live with. It explicitly states the Sith nature of the faction, bringing the story of the saga full circle (back to the essential Jedi-Sith conflict) and it doesn’t recycle either Empire or Order. But I think maybe some dialogue tweaking could serve to further establish that the First Order is either subservient to, or merged with, the Sith Eternal/Final Order. The distinction between the two organisations is not very clear when taking only the film as reference.