change the $content_width

Is there are better way to change this (annoying) variable please?

The Twenty Sixteen theme provides you with the twentysixteen_content_width filter, where the default is 840.

To modify it you should be able to use this within your child theme’s functions.php file:

add_filter( 'twentysixteen_content_width', function( $content_width )
{
    // Override the default 840 content width
    $content_width = 2000;

    return $content_width; 
} );