Hi,
Is there an API (or website page) that would allow me to upload all my deck collection at once ? I mean not just my inventory, but all the decks with title, description, cards (with sideboard).
Deckbox would only be an option for me if I can do that, because importing this amount of data is not possible manually.
Thanks!

You can import a card collection with a CSV (coma separated Value) spreadsheet or a basic text list as long as it conforms to the import syntax. You can do the same for decks as well but you will have to to each deck separately.

Hope that helps