Zombie AI Mechanics

Amazing idea, ill keep it in mind for future projects since I’m not at that level of coding yet.
But from what I understood, the server would only load parts that would slowly pathfind towards the player, and all animations & character physics would be handled locally?

Yes, you have the right idea. I was thinking of doing this for Zombies are Attacking RoDonalds but did not quite get around to it.

Letting the server do the physics & collision on the NPCs can be a big problem thanks to replication.

I believe Roblox physics sends data packets to clients for replication, you can test this out by using the performance stats in-game and looking at the ReceivedNetwork (Recv) tab.
I could be wrong, but this is coming from a personal experience making NPCs that attack players.

I agree with the NPC representing one part on the server, this will help a ton with projectiles damaging them. I’m currently making a NPC system myself, I don’t render bots who are not in my view by using dot product.

Shortened; The more NPCs, the more queue & data packets being sent to clients if you’re using Roblox physics.