Looking into the fastest way to drop pieces for trees, on Level 18 (drop speed = 3 frames), holding Down (which increases drop speed to 2 frames) is equal to doing 3 Down, 1 blank (which alternates 3 frames and 1 frame). Though you want to start that Down hold one frame after you gain control of the piece, to save 1 frame. Since 3 on 1 off is equal speed to holding Down, horizontal movement can be done in the off frame (as well as the very first frame) without losing time.
But 5th-8th knee will be getting cleared on Level 19, right? So if you want maximum potential frames, they have more limited hole options. (In reality, the extreme difficulty of manipulating the I-pieces would almost definitely lead to RNG delay frames, during which you can do horizontal movement freely, but I figure I should bring it up)
Bottom-most hole is column 5/6/7 for early input end (probably 6 for most options)
Next highest hole I believe has to be 3-9.
And above that, it'd have to be 4-8.
Hmm, yeah, it'd have to be two J's... but what about this?
I wonder if you could look into some larger 5-line options like this... (Just realized I didn't test that its actually possible to get the pieces there, especially that T on the far right, but I'm sure there's some similar shapes possible)
(If we get too into this, using Discord for real-time chatting could be more useful than swapping replies on the forum here)