Scripting For InDesign To Do Several Actions

Yes - possible - does this work for you?

// Step 1: Scale selected grouped objects to 81%
var doc = app.activeDocument;
var selection = doc.selection;

if(selection.length > 0) {
    for(var i = 0; i < selection.length; i++) {
        var item = selection[i];
        if(item.constructor.name === "Group") {
            var scaleFactor = 81 / 100; // Scale factor for 81%
            var itemBounds = item.geometricBounds;
            var centerX = (itemBounds[1] + itemBounds[3]) / 2;
            var centerY = (itemBounds[0] + itemBounds[2]) / 2;
            var transformMatrix = app.transformationMatrices.add({horizontalScaleFactor: scaleFactor, verticalScaleFactor: scaleFactor});
            item.transform(CoordinateSpaces.INNER_COORDINATES, AnchorPoint.CENTER_ANCHOR, transformMatrix);
        }
    }
}

// Step 2: Change the document Document Setup from 297mm x 210mm to 148mm to 210mm
doc.documentPreferences.pageWidth = "148mm";
doc.documentPreferences.pageHeight = "210mm";

// Step 3: Centre to page vertically & horizontally
var page = doc.pages[0]; // Assuming you want to center on the first page
var pageBounds = page.bounds;
var pageWidth = pageBounds[3] - pageBounds[1];
var pageHeight = pageBounds[2] - pageBounds[0];

for(var i = 0; i < selection.length; i++) {
    var item = selection[i];
    var itemBounds = item.geometricBounds;
    var itemWidth = itemBounds[3] - itemBounds[1];
    var itemHeight = itemBounds[2] - itemBounds[0];
    item.move([
        (pageWidth - itemWidth) / 2,
        (pageHeight - itemHeight) / 2
    ]);
}

alert("Script completed successfully!");