Hiya,
Thanks for the feedback - I genuinely really appreciate it (and also the bug report - I've uploaded a corrected build for that).
You are probably right about the amount of health the enemies have being a little on the high side - I was actually have pretty serious performance issues on actual hardware for a lot of the development of this until I figured out that localising the Trig maths functions would cut down the CPU usage by literally 40%, so I ended up dropping the number of spawns and nudging the HP up a little. It gets a little hard to figure out if you have the balance right with this stuff once you've played through it enough to autopilot through it due to knowing where things are going to spawn. Live and learn, I guess - as it is, I've actually got pretty significant overhead for throwing more on screen at once now.
I'm not too sure if I'm going to do much else with this one at this point - I'm already feeling a little bad that I put my other project to the side for long enough to work on this one, and I'd probably need to come up with at least another 4-5 enemy types and a couple of extra background visual gimmicks (plus some new bosses), and maybe some actual powerups, for me to consider it as being worthwhile. It's certainly something I've been thinking about, but I do have some other stuff to try and finish before looking into it.