Invaders might not need to bring anything with them at all. Particularly relevant to your point, I'll share one of the military projects from Sid Meier's Alpha Centauri: https://www.youtube.com/watch?v=aKcEwUcVBHs
More generally, the point I'm raising is symbolized in entirety by the Spartan faction's ideology in that narrative environment.
You still lose to the first-mover advantage though. If universal assemblers are possible, then the defender with the resources of a star system at their disposal and early warning of the attack is going to outproduce the aggressor.
The only advantage I imagine an invader would possess is the possibility of taking control of the Lagrange points and any orbital production facilities, and then besieging the population below.
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.
That assumes the Lagrange points are empty. A civilization capable of interstellar war is going to have a lot of space based assets.
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.
> On top of this the only thing worth shipping between stars is information at the speed of light.
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.
More generally, the point I'm raising is symbolized in entirety by the Spartan faction's ideology in that narrative environment.