Good idea. I feel like a combination of the last two would be most efficient. Low drop rate at start, as more monsters are killed drop rate increases, and if no rift stones drop on the floor, the last mob is guaranteed. Another option would be to take the same approach the devs took with the trophy...
There doesn't seem to be a lot of attention paid to the very beginning of the game, which is where I need the most help. https://i.imgur.com/AF2uI49.jpg The vanilla portions of this game can be facerolled almost blindly - which is why you will not see much discussion about the beginning of the game...