I'm a little confused about the rating at the end. It seems like, if you beat the game quickly, you get a lower score because you didn't buy upgrades or get the unique characters that don't show up 'till later.
Similarly, if you win the league before conquering all the cities, it doesn't let you challenge the king - which, according to the intro you ought to be able to do.
Having played through the game a bit:
There is a huge range of usefulness variation between the various upgrades.
A bit of spoilage here:
Cooling is by far the best thing to upgrade since it lets you go lower.
Vibration upgrades make it harder to get lava or do other 'vibration sensitive' things - I now skip them
A level or two of optics is nice, but most of the other upgrades don't seem to make that much difference.
The 'bonus time' you get is effectively endless.
The 'fuel can' consumable seems to provide less fuel than what you'd get for converting the raw materials, is there a reason it doesn't just completely fill the tank?
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 "The King's League"){kind=icon}
 "Glean"){kind=icon}