It's an awesome MVP! I played a bit and it's got a good foundation. Only two comments:
1) Weapon reload time should be faster (since you can't stay in the location very long before getting swarmed)
2) Even slower slowdown when selecting function.
If you make it easier now and focus on making it fun, then when you want to up the difficulty you can revert those two things.
Just my opinion. Hope it was helpful!