Since you decided to go ahead and buy the DLC at full price, you must have thought it was worth it at the time. Has your opinion changed about the value of the DLC just because it is now on sale? You know video games inevitably go on sales every now and then, that shouldn't change the intrisic value that you subjectively give them. If you're feeling remorseful and suspect you might have overpaid for the DLC at full price, maybe you shouldn't have purchased it then?
If you think the HOI 4 DLCs are not worth the full price they're being sold for, like I do, and would prefer to wait a little bit and take advantage of periodic sales, which happen once per quarter on average, may I suggest you use Steam's wishlist feature. You can add any game or DLC to your wishlist and be automatically notified by Steam via email whenever it goes on sale in the Steam Store.
For tracking prices, sales, bundles and discount coupons across a wide range of online game stores in the region/currency of your choice, including the Paradox Store, I'd like to recommend the IsThereAnyDeal service (
https://isthereanydeal.com/). It's a free service, you can sign in with your Steam account (no need to create a separate account). It allows you to create a list of games and online shops to monitor (the game list can be synced with your Steam wishlist), and to set a "target" price or discount for each game. The service then notifies you when that target is met. I have no affiliation with this service, I'm just a longtime satisfied user.