Well, think back to the Black Ops II Zombies mode. They had rounds, where a set number of zombies would spawn every round. Those would be the waves in this game. Also, this is a video game. You really don't need a whole lot of logic to make a good game (Slightly off topic).

So, I might have a better idea: Why not combine Detonator mode with Zombie mode and design a new ship for this mode (Unlockable) that can reload on the spot?