Ordinary Banana

why invade vs destroy?

To some extent they're the same thing but destruction is even more expensive because you have to send enough energy to kill everyone there

with invasion you can send von-neumann machines that can slowly build up to take over that solar system

Hence why I said at similar tech levels. If you can make planet killers they can detect and deflect them for cheaper.

if they're going fast enough you won't spot them very far in advance