I think that a cool addition would be some type of notification (lit up segment of the icon or some such) that lets you know when you have enough xp for an upgrade without going into the skills tab and losing your mission multiplier.
I like the idea behind the game, but it is not very balanced. The way it looks without doing a lot of math is that the planets in the first solar system are the only decent place to spend your energy. The progression looks linear so you get more per energy spending it on the last planets in the first solar system with some moon upgrades than on anything else. I am at planet lvl <2000 in the first system and I am making like 99.9% of my income there and it is still cheaper to upgrade those than the ones in the third or forth for much less gain in the later systems. Still give it 4 stars for being at least a good effort. I know balancing is hard in these games keep trying.
When I finished the level the money indicator in the bottom right corner of the screen said $5722. After the level has finished and it gave me the bronze trophy the money indicator at the top only has $4572. What happened to my other money? It has happened on more than 1 level, this one I just got the numbers for.
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 "Astro God"){kind=icon}
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Thank you! Yeah, it was so hard to balance this game, and even after lots of changes it's still unbalanced