I absolutely loved this!
One question: the reward version is, in some ways, more restrictive than the free version, because the CC BY-SA license forces the user to use the same license in derivative works. So, for instance, if I make a game using these assets, my game - even if sold commercially - would still have to use the CC BY-SA license. That means whoever purchases it is able to freely distribute it. On the other hand, with the free version I could distribute - but not sell - my game using any compatible license. Is that intended?
If not, the simplest alternative would be to use the CC BY license instead, which does not have the "share alike" clause while still requiring attribution.