Which of deluxe expansions would you recommend to boost the game in the best way ?
The first expansion I would buy is Edge of Darkness. It will complete the starter decks for the Scum and Villainy and Smugglers and Spies affiliations.
Star Wars is pretty unique for a CCG-type game in that deck building can be as simple or as complicated as you want to make it. The objective set system really does make it very approachable.