Hi. There is a greek website which should have close to 100% of the greek books with an isbn. It's https://biblionet.gr/
Maybe you can consider it as an additional source if it's possible. If you add it please use the greek website because some of the books in the english one (https://biblionet.gr/?lang=en) have translated information (mainly some publishers afaik).