Quarries.
How can quarries be an efficient base defense, you ask? Well, you only have to use enough of them. And then have your small, well-lit base floating above the hole. At the moment, the closest edge of the hole is more than a hundred metres away from the base. The farthest edges are so far away they unload from memory.
Should a mob spawn in the base anyway, such as Pigmen appearing near Nether Portals, my trusty Snow Golem will deal with them. Since the base is located at Y=80, and the hole goes down to Bedrock, the gentle push of a snowball will prove very lethal to any invaders.