I hardly ever use intros--in fact, can't think of any I use. I do use intro 1 for a additional "break" very often.
Upon starting a song, I automatically open the intro-ending screen and select intro one, and usually ending one before starting the song.
There are certain songs when I use the other endings on certain styles, but I always play on top of them.
The lack of dedicated buttons for all intros and endings is the single worst feature on the 2K-21K. Even at that, it's better than the Korg PA80s intros/endings.
At at least the buttons are where you can reach them with the left hand, unlike Solton X1.
DonM