Wow big improvement from last I played it, it's really fun, you can make really overpowered builds but there's still tension since you can die anytime if you fail the bullethell. Couple of things though, I was wondering why do bosses give 0 gold and exp, felt kinda weird. It would be nice if there was a small help screen to explain stats. As for the skills and items having a tooltip when you hover over them that tells you what they do would be cool, especially for the upgrades shop since you currently can't check your current items stats so you don't know how how good the upgrade is. I tried a couple of runs (3) and they all felt pretty different with all the choice of skills, skill upgrades and items. Speaking of, the skill upgrades are pretty much build-defining but you don't see which ones you currently have, I think you should really have them visible somewhere, even if only inside the skill tooltip.
Oh and by the way in the last stage you can see the grey skybox if you go to the bottom left, bottom right and top left.