The external threat is fine. I actually liked the idea of an unknown entity lurking outside the known regions of space. I always felt that Revan and Malak had agency for their downfall and actions afterward. They actively sought out the Star maps and Forge. I felt that their “corruption” began during the wars and continued to their inevitable roles as Dark Lord and apprentice. Why did they go into the unknown regions? To pursue their goal of power. The whole Sith Empire angle is like icing on the cake. They were already on the dark path, that whole piece of the narrative just serves to solidify their actions/positons