The "secret" is to wait to place a turret until you see the first astroid, and then be a little lucky with the spawn so that you can manipulate the turret rotation until you get enough credits for the second turret without loosing too much health.
But once you get ahead of the curve, it immediatly becomes to easy and you can fill every slot on the station.
So, yeah, balancing is definitly an issue. In general the entire game is a bit... unwieldy when it comes to the user experience.
Yeah, the "-1" bug was a '==' instead of an '<='.... already fixed on my local copy of the game :D
So, thank you! i do currently evaluate what i can do/fix/add with little effort, and what needs more effort. Plan is to at least adress the balancing issues with the current setup after the jam.