The spectrum of cultures tend to include just about anything you can imagine - and a few thing you cannot. You might want to ask this question of /r/Askanthropology since it is more a question about cultural assumptions than about history or even folklore. I don't know enough about Finnish cultural assumption to address your specific question about the Kalevala, but we have to remember that although that epic poem of the Finnish nation draws heavily on folklore, it is not a pure presentation of oral narrative: it is modified and messaged into something else. To verify that the Finns think of the sun and moon in the terms you describe, I would need to go to actual ethnographic work on the Finns - and I don't have that at my fingertips.

To me, it seems that the sun is generally kind of yellow and the moon is kind of white, so I suppose it is not surprising that they could be associated with gold and silver respectively. Alchemy may have nothing to do with a core cultural assumption - in fact, I doubt it drove any engine that caused a core cultural assumption; rather, alchemy tended to respond to those assumptions.

The real question - again best presented to the anthropological community - is whether there are cultures that reverse that core assumption about the sun and moon being associated with metals.