would there even be a dark age?
By the time of the Frankish empire Europe's already in the dark ages.
Strong rulers aren't always a good thing. Powerful monarchs usually end up trying to expand their territory and the Frankish empire was already too vast to prevent the Majordomos from stealing authority from the Emperors.
This actually leads to the empire growing weaker, and its one of the resons the Merovingian dynasty collapsed.