Thanks for the responses. I’ll keep the ship design elements you mention in mind for dealing with it.
I saw news that it has moved on to attack another race again, and that made me start to wonder about a new concern: does this thing just keep roaming the galaxy destroying planets until someone kills it?
I probably should have but in a spiral galaxy like this one the point I encountered them they were already ahead of me. So I took out the two races at the bottom of my starting spiral as they were targets of opportunity I felt like I had to move on before heading to the middle of the galaxy. Otherwise I invited a multiple front war and these ships don’t move across the galaxy terribly fast so I would have gotten myself stuck building ineffectually a fleet to fight on two fronts (and you know what they say about that).