You should plan to save if money is an issue; there is no advantage in terms of savings or price when preordering. Most preorder deals apply exclusively to games and offer incentives for preordering, e.g. the Purple Phoenix for EU4. The alternative is a season pass that offers all future dlcs for a total less than their individual prices, which would not be an entirely bad idea considering the rate at which dlcs are released for this game.
I doubt many people would preorder dlcs even if they were placed on steam early for that purpose. Preorders tend to exist to show that their is support for a game prior to release; while I do not speak for Paradox, I suspect that they are well aware of the size of their base for EU4, given that it has been nearly two years since release now.