I have deliberately tried and tried time and time again to crash my economy in this game and plummet approval rating really badly so that I could start a civil war on my own empire but every time my empire's either too powerful/developed or my population reduces from 1 trillion people to 20 billion and nothing happens. It's sad really, there's certain things GCII gets right way better than Stellaris. Part of your fleet defects to the other side and it really feels like a civil war on equal terms.
This other time I conquered the Drengin Empire without defeating all of their fleets first (they had the 3rd largest navy in that game) and all their remain ships just turned into pirates and split into individual ships. I spent twice as long trying to kill off all the pirates as I did conquering the Drengin. Those former Drengin Navy pirates were also killing off all my civilian ships and trade ships too. It almost felt like I was fighting the space iraq war.