My posts keep disappearing for some reason. I'll try again.

"I know I started out by talking about keyboards, but now I think of it they aren't the only way to produce notes - preferably more than one at a time - with a percussive attack & decay..."

This may be stating the obvious, but the instrument you're describing is a guitar: portable, polyphonic, infinitely adaptable to virtually and style of music, available at all different price points. There's a reason so many people play the things.