Thanks for playing!
The difficulty is indeed all over the place, although I did want at least one game to give the player a change to catch their breath even on the highest speed (this was not supposed to be Diamond Heist though but... Beach Walk! LOL). Diamond Heist does remain the easiest (imo) and slowest (objectively) throughout the game, it was the first minigame I designed and balanced after all. I also ran into a problem when balancing the game, because I wanted to make sure that even if two holes spawned at the opposite ends of the screen the player could still win if he had good enough reflexes. At higher speeds this made the player move so fast that actually aligning yourself with a hole became almost impossible, so I had to turn the speed scaling down quite a bit (looking at the code it scales with the game speed at a factor of 0.5, whereas most other games have a direct scaling).
Cave Painting doesn't scale too well either. The other minigames do get quite fast, but even Beach Walk should be doable at all speeds. The screen warps around in that minigame, which can be quite effective at higher speeds. If you get the angle right you can just let the game play itself, as the player will eventually cover the entire screen x)!