The hard mode is not much different from other modes.
The few differences are the number of enemies and the types of them.
But... maybe yes. There is an "endless mode".
Rounds 8 and 10 in hard mode are actually very difficult because of the amount of enemies, and if you don't blow up the blimp fast enough, your PC will start screaming.
If you want something like "keep the enemies coming! until i explode," you could start a wave with a blimp and don't blow it up.
Theoretically, the round would never end.