I really like the concept!
For more replayability you could add a point system, for each zombie you have "created", and with those points have improvements or other maps.
Another thing that I have seen is that when I lose all the zombies, nothing appears on the screen as I have lost ...
If you continue the project let me know;)
Ah, if you want you can try my game: https://aitramkg.itch.io/water-together