Complex sentence: In nearby Philadelphia the elite Proper Philadelphians were rich, charming, tolerant, but had relinquished the role in governing the city thereby making it by common agreement, the largest and most cosmopolitan but also the most poorly governed city in the Colonies.
Split into simpler sentences: In nearby Philadelphia the elite Proper Philadelphians were rich, charming, tolerant, but had relinquished the role in governing the city. Philadelphia, by common agreement, was the largest, most cosmopolitan but also the most poorly governed city in the Colonies.