Thanks, I'm glad you're enjoying the update!
As for the suggestions, I think the line clear delay is pretty good as is. It strikes a good balance of encouraging multi-line clears while still not punishing you too much time-wise for single line clears. Most other popular unofficial Tetris games omit line clear delay entirely for the sake of speed, but I mostly put it in the game because it adds a lot of character to the visuals.
Speed wise, the game has the guideline values, so it's exactly as fast as Tetris DS. But Apotris lets you change DAS and ARR, making moving pieces to the edges of the board much easier. I don't think adding infinity (no step reset limit) makes much sense, it would make the game way too easy. Try changing Auto Repeat Delay (DAS) and Auto Repeat Rate (ARR) in the handling settings, and see if that helps.
I checked many times if there are any more bugs with piece locking, but I couldn't notice any. I think it might be that you're too used to infinity... Also from the tetris wiki: "Most games since 2007 have a limit of 15 move resets before the piece locks."
Lastly, while the game might be "difficult", it's way more important that it's fair. That way, you can practice and become better!