Almost three months later, here's the promised rule system as well, at long last:
https://notimetoplay.itch.io/gng
I don't have much more to say about it that's not already on the game page, except I left out many nuances from The Unwanted Hero that would have been hard to explain briefly and are specific to that game anyway. On the plus side, I managed to keep the text at a length of 1000 words. Beware, it hasn't been edited much.