Terrorism, as you mention, might be possible (but again remember, no stealth) and claim jumping (where you displace another group that got there roughly when you did). But even in densely packed stellar neighborhoods the average star is surrounded by a moat several light years across. Once the locals are developed enough to mount a serious defense, they're probably immovable.
More generally, the point I'm raising is symbolized in entirety by the Spartan faction's ideology in that narrative environment.
Dropping asteroids on habitable worlds and causing ice ages to clear out the population _might_ still be faster/cheaper than terraforming a semi-hostile world.
Further starship drive capable of reaching a significant fraction of the speed of light make one hell of a weapon. But you still need to deal with space based assets that can dodge your initial bombardment and at a minimum are armed with powerful engines. So sure you can destroy planetary populations etc, but now your still out gunned 1,000:1 locally and pissed off a civilization capable of doing the same thing to you. Net result MAD.
A large technical advantage may make up for this, but that means the winner has better tech is much harder to take over a second time. On top of this the only thing worth shipping between stars is information at the speed of light. Making conquest largely pointless without FTL.
It could be worthwhile to beam energy out to interstellar distances (also at the speed of light). This is probably needed for interstellar travel anyway.