Admin Options page. Save as Array

You only need to register one setting and then just modify your form inputs to save the values into an array. Here’s an example of registering the setting:

register_setting( 
    'mytheme_settings', 
    'mytheme_settings', 
    'admin_options_sanitize' 
);

$mytheme_settings = get_option( 'mytheme_settings' );

and the field markup:

<textarea name="mytheme_settingsAdmin Options page. Save as Array">
    <?php echo esc_textarea( $mytheme_settings['title'] ); ?>
</textarea>   

You’ll also need to combine your sanitize functions, modifying them to work with the values in the new array.