I'll state an old classic that is seen as a genre defining game because it is: Myst. Yes, it redefined the genre... in ways I fucking hated and that the adventure game genre took decades to fully recover from. It was a pompous mess in its presentation and was the worst kind of "doing action does vague thing or nothing at all, where is your hint book" puzzle gameplay wrapped in graphical hype which ages pretty poorly as far as appeal qualities go.
So many adventure games tried to be Myst afterward that the sheer budgetary costs and redundancy of the also-rans crashed the adventure game genre for years.
Hearts of iron 4 and Victoria 3 are far worse then their predecessors and deeply flawed because of the extremely boring tactics gameplay(frontlines). Victoria 3 economy is also extremely bland and feels like a auto clicker game. But, I only enjoyed vic 2 with pop demand mod which current vic 2 modders/players don't like.
I get your point about warfare being incredibly simplified in HoI4 v HoI3, however, I think that virtually everything else about HoI3 is borderline unplayable due to how many factors of high-level industrial economy it is trying to simulate. And I say it's unplayable, mainly because it's unreadable. I, as a new player, have no real way to figure out at a glance what effect any given action I take is going to have. And that makes it both harder to learn at all, and makes me not want to bother trying.
I think darkest hour is the peak of the hoi series. I wasn't a huge fan of hoi 3 but I yearn for its supply system compared to hoi4s oversimplified mess