Don’t worry about it hehe.
Can I recommend you increase the pace of the game, greatly? It escalates too slowly imo, also here is an idea:
So at the moment it gets hard, they spawn faster?
Give your zombies health, say default health is 1 and it increases slowly, slowly slowly as the spawns increase. Make your bullets have a knockback and do like 5 damage this should be fairly simple. This means as the spawns get harder, zombies get stronger, you have choose to shoot which side wisely, keep they away and kill them.
If that makes any sense, that imo will massively increase the challenge.