In the Setting it up for an eZDFSFileHandler documentation page, the clear cache command

$php bin/php/ezcache.php --clear-all --purge

Should already come in prepared form for symfony legacy for 5.1 =<

php ezpublish/console --env=prod ezpublish:legacy:script bin/php/ezcache.php --clear-all --purge

Also, the page should specify if it's necessary or advised to clear symfony cache before clustering

php ezpublish/console cache:clear --env=prod