Hey,
I played your game multiple times, that's just how fun it was.
At the start, I didn't realize what the extra healths were for, so I just used them as soon as they were up. Once I hit the 5000 mark I realized my error but there was no turning back :D
I also kept spamming space since it didn't have a cooldown, meaning that I had a permanent speed advantage :p
My only complaint would be that there are not more ships spawning on higher difficulties, meaning that at some point, since the spawning of the ships seems to be randomized, if you have bad RNG, there are only 2 ships spawning when you're on a difficulty that needs at least 3-4 to survive, but that's hardly addressable for a jam game so dw.
Cool music, cool visuals, fun, simple, topic respected, wowie!