Thanks a lot for this comment ^^ It made my day.
I will try my best once the jam is over to address some of those issues :)
Regarding the weakening of the void, new void units do have lower hp and attack power if you have more crystals. My original idea was to have different units that would be spawn depending on the percentage of void and at different rates, but in the end I feared that would be too hard to develop and more importantly, to balance.
Regarding the extra time to plan before the enemies attack, I totally agree, but only noticed was a problem after submitting though.
Anyway, I'll try to post a new version of the game once the jam is over. Would love to have some more feedback then :)