Complex sentence: Poniatowski was an "enlightened" Polish magnate who had been a deputy to several Sejms between 1750 and 1764 and had a deeper understanding of Polish politics than previous monarchs.
Split into simpler sentences: Poniatowski was an "enlightened" Polish magnate and had been a deputy to several Sejms between 1750 and 1764. As such he had a deeper understanding of Polish politics than previous monarchs.