I think you overuse the word impossible dude.

I mean, you assume that smashing everything in genocide is the only effective means to wage interstellar war. Why? I could counter that by simply saying that you can send out a counter RKV and blow up such easy to detect [since serious, that RKV going to flare] death weapon. I've also seen people who said RKVs are impossible since they'd move too fast and the costs for making a RKV would be comparable to that of making a interstellar colony craft. I think interstellar genocide would be hard to achieve against another interstellar civ since they'd intercept them. Instead setting up a base and building more invading spaceships in the outer regions of ther system could prove a more effective means of subjugation instead of sending a few big rocks and hoping for the best.

RKVs are to me only good for strangling civs in the crib.

And the trade/interstellar society stuff all assumes that the current pace of time we have will be the same pace for humans in the future. Perhaps humans, in order to counter the issue of long times to travel between stars simply start perceiving time in a way that makes a 45 year time only a week or two? And are you sure innovation is a constant that may not level off? technologically historically has been booms and busts, why would it be any different in the longer term future?

And besides, trade doesn't just have to be tech. Culture, biological resources [which would by the way, be quite rare in a cold, dark cynical hard sci-fi setting] and simply people who want to go somewhere else would all be involved in some form of interstellar trade. Impractical, perhaps, but can we assume that practicality will even matter to humans of the far future, seeing that the only factor is really time to make things and transport with all those raw resources from asteroids?

A interstellar empire I dunno about, but I could imagine a spaceship chain connected to a neighboring star system being subjugated. Yet with so many interstellar space craft required for a subjugation chain I don't think interstellar empires would be that big, like a couple dozen star systems at most. Of course, I may be absolutely wrong.

And as for the ramming, whatever man I think my theories on it are valid enough and what stops the smaller craft form being built in a similar way to a missile? Most a spaceship is not the crew as is.