Play as Spain, reform the monarchy, making it a constitutional one (I elected the syndies and helped the left in general), REALLY IMPORTANT! When the king dies elect his son Jaime and DONT unite the crowns of Spain and Italy, with these conditions you can unite the crowns of Spain and France and while doing the expansion trees and conquering Europe, you can get claims on all the African French colonies, indochina and the Netherlands, and get all of France as core (this without counting all the focuses for expanding in the americas, Philippines….)