dc42:
That's the problem then. There is no way that a serial port sending ASCII codes can convey information about how long a key was pressed. I think a Leonardo could do it, because I believe it can emulate a USB PC keyboard, for which the up- and down-codes are sent separately.
Problem with that is that the Leonardo doesn't have enough analog pins.
Do you know how I can fix the issue where your code works flawlessly in the serial monitor but starts glitching when I turn on aac keys?