Very very creative and simple idea, I love it and am very jealous I did not come up with this! One thought I had -- if 12/20s only completed at the end of the round, it would be much easier to do combos, which would make the game a ton more interesting imo. Just for fun, I spent a while figuring out a combo where one piece would get me 3 12/20s, only to realize that the random ordering of the dice forced me to score 2 of them before I could set up the third. If the 12/20s only resolved after i placed all my pieces, this could become a much more viable and interesting strategy.