"I might have gone a bit overboard with the cat count but I wanted to make the town feel alive." No, I agree with your intentions, it really aids the experience of the game having NPCs around the whole map! If my machine is just an outlier then it shouldn't be a problem, but if it's something several people bring up, the only thing I can suggest is maybe unloading/reloading models that aren't in the immediate view of the player. I think making assets outside of the player's view invisible is something Unity does by default, but it still runs the code for them.
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In unity it is called Occlusion Culling, only stuff in the camera view is rendered, it is not enabled by default but it is built in, just adding a few objects, changing a few settings and pressing a bake button. I set it up and baked the occlusion culling as well as baked the scene lighting, I am sure there are other ways to optimize the game, but I just can't think of them at the moment. I guess pausing the AI movement calculation if they are beyond a certain distance from the player, but tracking the distance to each cat from the player might add to the issue as much as it fixes. Definitely worth testing though.