And, of course, the other side of this is that everyone in the universe will know about all the threats from standard technology, as their cultures have been living with it for thousands of years. What countermeasures (technical and tactical) have evolved over that period?
Just for example, a run-up to a significant fraction of c is going to take much more than a week. So if you dot the Oort cloud with sensors looking for suspicious objects accelerating into the inner system, and give each that's more than a light-week out a jump-capable message torpedo, you can get word to the main world within a week, and a week later have a fleet arrive via in-system jump at a position where they can intercept the intruder at its predicted position two weeks after detection, and destroy it before it's within 50 AU of its target. Heck, jump a few 12G missiles with good homing systems out there, that will do the job just fine.
Obviously, that takes a lot of money and hardware. But the Imperium and its worlds have a lot of money, hardware, and time. Don't you think there's a defense shell like this around the major hi-population worlds?