Is the expression 'early in the piece' (earlier on in the event), considered to be an idiom?
I've never heard it used in any way other than to talk about a musical piece. Early in the piece, the main theme becomes evident, but then fades away, to reappear as the first movement is reaching its crescendo, or something like that. It's not an idiom when used like that.
Have you heard it used to refer to events other than music?
[not a teacher]