you should be able to use this hook which fires just before an auto update:
do_action( 'pre_auto_update', $type, $item, $context );
$type can be ‘core’, ‘theme’, ‘plugin’, ‘translation’
a specific solution would depend on your particular backup plugin/service, but as long as this plugin/service provides either suitable hooks or an API it should be doable
more details on the hook here:
https://developer.wordpress.org/reference/hooks/pre_auto_update/