Query posts order by multiple postmeta

Some of my posts will have two meta_keys set called ‘meta_1’ and ‘meta_2’ while most will only ever have ‘meta_1’.

‘meta_1’ will always contain a number greater than 0
‘meta_2’ will either be set with a number 1 or will not be set

I’m trying to get all my posts from the database queried in such a way that posts with ‘meta_2’ are shown first with all posts ordered by ‘meta_1’.

What I have so far is getting posts ordered by ‘meta_1’ in either ascending order or descending order or posts ordered by ‘meta_2’ being set or not set.

add_action( 'pre_get_posts', 'get_posts' );
function get_posts( $query ) {
  if( !is_admin() && $query->is_main_query() ) :
    $query->set( 'posts_per_page', '9' );

    switch ( $_GET['sortType'] ) :
      case 'ASC':
        $query->set( 'orderby', array( 'meta_value_num' => 'ASC' ) );
        $query->set( 'meta_key', 'meta_1' );
        break;
      case 'DESC':
        $query->set( 'orderby', array( 'meta_value_num' => 'DESC' ) );
        $query->set( 'meta_key', 'meta_1' );
        break;
      default:
        $query->set( 'meta_key', 'meta_2' );
        $query->set( 'orderby', array( 'meta_value_num' => 'DESC', 'title' => 'ASC' ) );
        $query->set( 'meta_query', array( 'relation' => 'OR',
          array( 'key' => 'meta_2', 'value' => '1', 'compare' => '=' ),
          array( 'key' => 'meta_2', 'compare' => 'NOT EXISTS' )
        ) );
    endswitch;
  endif;
}

How can I show the posts with meta_key ‘meta_2’ on the top of the query results and have all posts ordered by meta_key ‘meta_1’?

Say I have 4 posts (p.ID 1, 2, 3, 4)

• Post 1: ‘meta_1’ = 9
• Post 2: ‘meta_1’ = 7, ‘meta_2’ = 1
• Post 3: ‘meta_1’ = 3
• Post 4: ‘meta_1’ = 2, ‘meta_2’ = 1

Posts should be returned as:

• Post 4
• Post 2
• Post 3
• Post 1