When I was pondering ATUs/MTUs I had two main factions in the Solomani Movement, the Supremacists and Nationalists.
The former wanted to regain pre-eminence in the Imperium and were based around the Core but the Nationalists in the Rim just wanted to be out.
Someone should write a book about the realities of Terran space.
1. Terra moved its warfighting logistics during the later ISW era to the worlds that had been colonised or taken from the Ziru Sirka.
2. The Rule of Man didn't really affect Terra, the world governments of Terra, the USA, India, Turkey, China, the EU all had space industries, merchants, explores and colonists that had nothing to do with the UN and its Teran Confederation descendents.
3. The nations of the Earth and multinational corporations established their own pocket empires totally separate to the Ziru Sirka now under Terran Confederation rule
4. The Terran Merchantile Community replaced the confederation after the fall of the Rule of Man, with coreward states such as the Easter Concord and the Dingir league replacing the now defunct Imperium, while to rimward, spinward and trailing Terran exploration, trade, colonisation and establishment of new polities continued. The TMC would eventually become an alliance called the Old Earth Union, but we know little of its details.
5. While the Ziru Sirka collapsed into the Long Night the region to spinward, rimward and trailing of Terra continued to develop until recontact.
6. When the Third Imperium came calling they faced a loose collection of polities that they encouraged by fair means or foul to join, if the Terrans had said no the Imperium lacked the resources for a shooting war, especially after the Julian debacle.
This is what has never been explored in enough detail, all those pocket empires the people of Earth developed.
The Imperial story of the people of Earth mostly did very little during the long night until "the Solomani" rejoined the Imperium is stuff and nonsense, there is definitely more to it.
We know Terran merchants and explorers contacted the Aslan to Spinward and the Hivers to trailing, they even fought a war with the Aslan. The Sword Worlds were colonised by people from Earth long after the Rule of Man ended, The Turkish Itzin fleet made it all the way to the Darrians.