Answering my own question: for Oanda there is a very nice script from Kalmar and Moris in this thread: https://opserver.de/ubb7/ubbthreads.php?ubb=showflat&Number=480372&Searchpage=1&Main=58480&Words=%2B+%2BAssetsOanda&Search=true#Post480372

However, I am still confused: thinking what I did above would also load ALL instruments from the Broker, at least I understood that from the manual.