Hello,
i thought of maybe making the upgrade system like a little minigame where you have to avoid the fails and brokens, like your cursor avoiding obstacles and as the upgrade level gets higher so gets the intensity of the minigame. Let's say you have +12 the speed and obstacles get bigger (like getting broken as an obstacle is really big) and that white stones give you like an extra chance maybe...? I also think purples and greens should atleast be a +2 instead of +1. I think they should be obtained throughout either the token shop, event boss and or maybe future challenge rewards