This seems to be conflating dispersal to multiple star systems and trade/regular interaction with the newly colonized planets. Given that it would take 40 years

This seems to be conflating dispersal to multiple star systems and trade/regular interaction with the newly colonized planets. Given that it would take 40 years to get a reply to a message, discussion wouldn't be the normal pattern - by the time you see an insult/aggressive message, there is a reasonable chance that whoever sent it is now either regretful about it, or dead, or, most likely of all, has completely forgotten about it. Colonized planets in different star systems are effectively alone, able to receive old news, and potentially get deliveries of new colonists or technology every once in a while, should anyone on another planet send some. For anything urgent, it would need to be dealt with without calling back home - whether that is disease, local war (e.g. within the star system), or simply resource shortages - there just isn't a suitable way to top them up from elsewhere. That same limitation means that taking war to another planet is unfeasible - even if you're just redirecting asteroids, the target planet would be able to see them coming, and, importantly, has had as long as you have had, plus the travel time of the rock, to master manipulating rocks in space. Similarly, while you could target computer systems (interstellar cyber warfare), the most recent details about their computers you have is dependent on the last message you received - computer viruses from 20 years ago don't work on modern computers without quite a bit of effort to make them run, and even if you get them to run, the effects are limited.