Wordpress meta_query order by meta_value then post_date

Question

I am altering a query using the pre_get_posts hook. I have a custom field 'watch_now_url' and i would like posts that have that to appear first then posts without the field afterwards.

I would also like to have the posts in post_date order.

I'm trying:

$query->set( 'meta_query', array(
    'relation' => 'OR',
    array(
        'key' => 'watch_now_url',
        'compare' => 'EXISTS'
    ),
    array(
        'key' => 'watch_now_url',
        'compare' => 'NOT EXISTS'
    )
) );

$query->set( 'orderby', ['meta_value' => 'DESC', 'post_date' => 'DESC']);

Posts with the meta_key are appearing first but then nothing is in date order.