A defense is only "nearly invulnerable" until someone finds a way to beat it, and if war's the motivation for a change in offensive paradigm, that doesn't generally take very long.
Any solution we come up with would eventually get exploited and defeated in a real war, so perhaps what we're working with is either something not actually designed for a real war, or something that isn't actually a physical defense. A combination of the below two factors might suit you nicely.
The False War
Most interstellar civilizations learn to make peace with themselves and each other, or they fragment or are overwhelmed by the ones who defend each other when things get serious. The universe might not be governed by an overarching alliance, but it is at peace, since nobody advanced enough to build spacecraft is really in the business of getting anyone upset.
Nonetheless, some species have war on the brain, so they need to vent steam by holding mock battles. The weapons used in these battles are minimal, and the victor is declared based on, say, who depletes their enemy's shields first, or who fought more impressively according to some third-party judges.
Perhaps these battles have real consequences in place of actual space warfare, like territory shifts or financial/resource rewards. Why wouldn't you destroy your opponent outright if you wanted this? Well...
Too Big To Lose
Spacecraft are extremely expensive to the point where only a small number even exist, and they're highly valued across the universe for being so versatile and mobile. Destroying one robs you of an asset that you sorely need more of, so combat, which is only done if you can track down a wormhole-hopping spacecraft (difficult and requires a lot of coordination), involves boarding actions and piracy rather than long-range weapons.