I think Redo of Healer started falling off after Keyaru dealt with the initial cast of antagonists. Once he got his revenge, he went for a more traditional quest which doesn't really feel unique anymore, and the new antagonists have definitely less emotional impact. There is still Bullet of course, who'll hopefully get what he deserves eventually, and I guess the way he dealt with the King was kind of a highlight of the more recent chapters, but it felt like the early volumes were full of these "exciting" moments, who have now become incredibly less frequent.
I think the reason why i don't mind this manga is going the way it is is because i don't see the story in the manga as a revenge porn fantasy, like from the get go is kinda clear where the end goal is (although with King Jioral it was kind of a decision gymnastics but it's still making sense).
(This is gonna be a long read so bare with me)
It's like a videogame story, it's been clear who are the bosses and that's it. Even (iirc) Keyaru contemplated a lot about what'd he do after he got his revenge and most of the times, again iirc, he'd just go somewhere quiet and live with his harem there. So even the concept of having new antagonists will weakened the manga imo.
I drew this comparison a lot to Goblin Slayer, both are not actually a hero/warrior/fighter but because both were driven by anger, by revenge, they became one. Keyaru and Goblin Slayer didn't want to do this but in their mind they have no choice but to be like this. While Goblin Slayer has a more positive and slow healing approach, Redo of Healer has a more conventional approach; an anti hero. All the good he did is not entirely because he's in it for the good thing, it's because he's selfish. Hell, even to some extent him wanting to save his harem is also a selfish thing from him. If, for an example, King Jioral turned out to be a good guy, there's a chance Keyaru will turn up to be a full time villain, depending how the author wants it.
Which is why this manga is interesting imo. So far, the story's been taking us with barely any help from coincidences and plot armors, every decisions were made are making sense, every character has some kind of a development. I think the only time when we can truly determine whether this manga is good or not is when Bullet dies and the world is saved. What's Keyaru gonna do afterwards? Is there gonna be more or is he gonna retire? Will he still alive or will he die? Imo that's when we know whether this manga is cooking or not. Personally, i think it's better to kill off Keyaru at the end but then again, it depends on author and whatnot.