For the author, if they revist this, they might look into 7-bag random generator. TLDR, most modern tetris games, including tetris 99 select a permutation of the 7 unique pieces every 7 pieces; this means even though Tetris 99 shows you the next 6 pieces, you may know more about the next pieces.

For example, at the beginning of the game, it shows you 6 pieces, but you can determine the 7th piece because it's the one that's not shown. The 8th piece could be any of the 7 pieces, but when you see the 13th piece, you know the 14th piece (and when you see the 12th piece, you know there are only two possibilities for the 13th piece).

Additional look ahead every so often might be helpful, given that they found

> When looking only 3 moves ahead, Jeff achieves a Tetris percent of 8.9%. When looking 6 moves ahead, he improves to 9.7%—nearly perfect!

I'd also be interested in seeing optimization for combos. Or t-spins; t-spins send 2x lines of garbage for each line cleared; which I hate, because I grew up before t-spins were encouraged. :P