Thanks for the review, it's great to hear you enjoyed the game so much!
You make a good point about the difficulty button, I will probably add a quick note in the tutorial or make it more obvious another way.
The game actually only had 4 difficulties until a few hours before I submitted, when I just thought "What if enemies just constantly spawned?" Believe it or not someone has told me they survived over a minute at Impossible difficulty! I've only gotten to about 30 seconds so far haha