Which WordPress hook fires after save all post data and post meta?

Try with post_updated and use $post_after object. https://codex.wordpress.org/Plugin_API/Action_Reference/post_updated

You can use something like this,

function your_custom_function($meta_id, $post_id, $meta_key='', $meta_value='') {
    if($meta_key=='_edit_lock') {
        // if post meta is updated
    }
}
add_action('updated_post_meta', 'your_custom_function', 10, 4); 

you can use this save_post hook with your function. change your hook priority to 100 it will give you updated post

add_action( 'save_post', 'send_mail_to_user', 100, 2 );