Dealing with the Aftermath of the Demon King

You know, I want to write a novel where the MC was the hero, but the demon king's influence still remains even after death as his presence has caused great suffering and turmoil and now he must focus on the small things that occurred because of his presence and shows just how ignorant he truly was to the suffering and how little he truly embodied the Hero, and I want to know of any novels or stories like that.

No, I don't mean Frieren, like actually widespread effects from the demon king's presence,

Well go for it, write your story sounds intriguing.

I'm not sure if these will be exactly what you're asking for but there's a few anime and light novels that touch on the topics you mentioned.

You could try death mage doesn't want a fourth life. While it doesn't have a mc hero, it does have a world set quite a bit after a demon lord invasion. You get to see how people misinterpret messages from their gods, how mismanagement split the gods and their followers, destruction of a whole continent. And a whole lot of other things. There are heroes in this one too and they also make their share of poor decisions or fail to comprehend the complexity of situations.

There's a few novels dealing with the impact summoned heroes have on the world like no longer allowed in another world.

There's also realist hero rebuilt the kingdom. This one takes place during a demon lord invasion and focuses more on the adminstration side of things