I'm not so sure if being a former True Sith would work, at least for me anyway. I've always imagined the True Sith being of the ancient Sith species, and of course that wouldn't be the smartest path to take. Seeing a person from the thought-to-be-extinct Sith species walking around the street would be a little obvious.
Now maybe if the True Sith had, years before, gone looking for new students, they could've found this person (you) who was naturally strong in the Force (again, a good explanation for this would be needed, and I don't mean "you were a former Sith Lord/Jedi Master who's forgotten everything he learned"), and taken him in. This would somewhat explain why your character would be powerful in the Force at the beginning of the game without having to do the whole cliched "start out as a Jedi who doesn't know he's really powerful" story.