The youtube video only just showed up in my recommendeds. This is such a solid hardcore multiplayer concept. I do have suggestions for the zombies: Reduce their respawn timer, but code their respawns to apply a random Teleportation Potion effect instead of just spawning them at world or bed spawn. I had to have help, but my skyblock mod messes with respawn mechanics, so it can be done. I saw there was a player tracker in the second video, but haven't seen how it works yet. I'm picturing the Boss Cursor mod. I'd make the player detection distance configurable. Then, disable NPC and Pylon map icons for zombies so bases are hidden.

Terraria has some serious imbalance in multiplayer, especially after hardmode. I think in order to be "thematic", zombies should be given extra health to make them less squishy than the average player, but then limit them to only true melee weapons (no sword beams, yoyo's, whips, or minions), the "safest" way would be to set all but a list of weapons/tools to 0 damage.