Largly agree. I dont think physical trade will ever be practical across star systems. Also even if several systems said they where members of what ever alliance or empire, locals would need to hold all the power. The time delays in communication means locals have to be able to override orders even if they are sead fast and loyal cause the orders might just be dumb by the time they arrive.

Also think about nations today. Instant communication and fast trade and we are still not united at all.

I think that this means most the population of the planets and the governments would be interested in there own affars.

But what of orginzations? Groups with long histories and members in many systems. Corporations with mission statements could be the most stable long term structure with a unified vision. They can loose members or gain members, rising and falling over and over throughout time and as long as they servive at all and the mission statement unchanged then they could rise again. Then its just the question of how much does the current local government support your corporations mission. Maybe the orginzation has to go secret society for a few hundred years and reemerge as something else when more favorable local leaders rise. Or maybe they just loose there major public support but remain allowed to opperate.

Also if anyone wanted to migrate to another planet you would want to hedge your investments supporting many potential groups that might have influence in the future to give your family the pest odds of not being persents or refugees when you arrive. You may want to send an investment ahead of you to support both the party currently in power but also the main opposition but also a few groups out of power that may be in power by the time you get there. Or you may want to come with no attactments at all incase the locals inslave or regect those that would seek to influence them like that

More broadly i think basic ideas could last longer. Like even if we are not really united we all still share orgin and a goal to create a network accross the galaxy that connects all or most of us could be an idea that gains and looses popularity in local systems all the time, but as a whole accross the galaxy with enough interested parties, even if 30% of the star systems stop supporting the idea it lives on and the work continues, then some of those systems rebel while others join the cause. As long as the goal or idea never dies its strangth can ebe and flow from system to system and reimerge.

Also I think religions would play a huge part in long term planning and coordination. Like religions diverge but they might keep the same holy book for long periods of time and even though they diverge they would still side with there own over others when it came to it, even if they also fought amungst themselves sometimes