Is there a better way to (Un)select all LegendItems in HighCharts LineChart?

In this case it is better to use setVisible method (called internally by show/hide). It gives a possibility to control chart redraw process. By using hide/show chart is redrawing in every iteration.

btn.addEventListener('click', () => {
    for (let i = 0; i < seriesAmount; i++) {
        chart.series[i].setVisible(undefined, false);
    }

    chart.redraw();
});

Live demo: https://jsfiddle.net/BlackLabel/f3L4oyqa/

API Reference: https://api.highcharts.com/class-reference/Highcharts.Series#setVisible