Is it just me or is it just the level I am playing on (Normal)?
But in scenario 1, the Drengin are almost entirely passive. Once you have cleared out their first two fleets, they don't attack you anymore but just sit back on their planets and bases, so you unmolested and easily gain resources and colonies and build up/rebuild your massive fleet and simply stomp their remaining fleets one by one.
They don't attack your starbases. They don't attempt to attack your colonies. They don't send fleets to slow you down. They just sit there passively until you have quietly build up overwhelming force. It basically turns into a end-turn rapid click-fest until you have the force you need.
Does this also happen on the harder difficulty levels?
Enjoying the campaign so far, but a bit disappointed in the resistance the Drengin are putting up.