Conditional Orderby

Conditional orderby

$order = '';

if ( sort_group = 1 ) {
  $order = 'ASC';
}

if ( sort_group = 1 ) {
  $order = 'DESC';
}

$args = array(
    'orderby'   => 'meta_value_num',
    'meta_key'  => 'start_date',
    'order'     => $order;
);
$query = new WP_Query( $args );

I'm doing this via pre_get_posts() and here is what my current query looks like:

$meta_query = array(
	'relation'    => 'AND',
	'status_sort' => array(
		'key'     => 'sort_group',
		'value'   => 0,
		'compare' => '>',
	),
	'date_sort'   => array(
		'key'     => 'start_date',
		'compare' => 'EXISTS',
	),
);

$query->set(
	'orderby',
	array(
		'status_sort' => 'ASC',
		'date_sort'   => 'ASC',
	),
);

// Set the meta_query parameter.
$query->set( 'meta_query', $meta_query );

I'm also looking at https://developer.wordpress.org/reference/hooks/posts_orderby/ as a possible option, but that's not getting me any closer just yet.

Really appreciate your input.