How can I put a custom meta box above the editor but below the title section on the edit post page?

Print your meta boxes out there, then remove it so it doesn’t appear twice.

// Move all "advanced" metaboxes above the default editor
add_action('edit_form_after_title', function() {
    global $post, $wp_meta_boxes;
    do_meta_boxes(get_current_screen(), 'advanced', $post);
    unset($wp_meta_boxes[get_post_type($post)]['advanced']);
});