Java Script for resizing the canvas

You can try the following code:

// artboard_create-new-A4-artboard-based-on-center-of-the-existing-artboard_0.jsx
// provided: There is only one artboard , centred on your entire artwork.

// regards pixxxelschubser 24-06-2023

var aDoc =app.activeDocument;

var ab_0 = app.activeDocument.artboards[0];
var abBounds = ab_0.artboardRect;
var ableft = abBounds[0];
var abtop = abBounds[1];
var abright = abBounds[2];
var abbottom = abBounds[3];

// calculate the new size based on the center of the old artboard
var abcenterX = ableft + (abright-ableft)/2;
var abcenterY = abbottom + (abtop-abbottom)/2;
A4_xHalf = 595.275590551180/2;
A4_yHalf = 841.889763779528/2;

abBounds[0] = abcenterX - A4_xHalf;
abBounds[1] = abcenterY + A4_yHalf;
abBounds[2] = abcenterX + A4_xHalf;
abBounds[3] = abcenterY - A4_yHalf;

// create the new a4 artboard
var ab_1 = activeDocument.artboards.add(abBounds);

// remove the old artboard if you want - or comment out the following line
ab_0.remove();

If that works for you
have fun

😉

Questions:

Is the (old) artboard always centred on the artwork?

Is it important to always bring the staves (the line grid of the musical notes) to the same position (in Y)? Because depending on the type of notes in the top and bottom staves, the position in Y will differ slightly on each new page! in this case: Is it possible to automatically isolate the staves in a separate group?