There is already a summer sale going on the Paradox store.
Paradox Interactive is a world leading PC games publisher known for games such as Cities: Skylines, Europa Universalis and Crusader Kings.
www.paradoxplaza.com
If the Paradox store is available for your country/currency/payment method, it's a good idea to purchase there, because it puts more money directly into Paradox's pocket rather than Steam taking a 30% cut. And you get Steam keys anyway.
HOI 4 is currently 75% off, and the older DLCs are 50% off. Expect to see the same discounts in the upcoming Steam sale (~ June 25 to July 9).
Don't expect better deals than that unless it's a bundle. DLCs sold separately never go below 50%.