Ah the Wilhelm scream! Nice game, I suspect given more time you'd have been able to implement a lot more to it. I enjoyed it and played until the end as it's compelling. I wanted to know what happens when you get all the humans. Definitely continue development on this, could make a great mobile game like hole.io etc!
An option to upgrade your zombies and add powers may work and a police force who hunts the player which gets harder the bigger the zombie horde. Especially, as there's a police station in the game. Many things you can do to this going forward!