Complex sentence: Antoninus tried to portray himself as a magistrate of the "res publica", no matter how extended and ill-defined his competences were: therefore the fact that he is credited with the splitting of the imperial treasure, the Fiscus.
Split into simpler sentences: Antoninus tried to portray himself as a magistrate of the "res publica", no matter how extended and ill-defined his competences were. He is credited with the splitting of the imperial treasury, the Fiscus.