Ty for the thoughtful feedback!! Heard similar stuff from other folks and after playing for a day or two I agree. I'd wanted clusters to improve dynamics in the first 30 seconds but they kinda stuffed up the late game. I've removed them now. The first few asteroids are just guaranteed to spawn close by instead, which I'm hoping will be the best of both worlds.
I'd be keen to here how the current version feels if you get the chance to try it.
Interesting mechanical suggestions too! Especially the idea of overheating. I quite like the simplicity of the game at the minute but if I'm trying to beef up the game in the future I'll try them out ((: