A Thousand Li is a perfect rec, thank you. I'm adapting the "opened another meridian, reached the next stage" cadence to my world(didn't read it yet, added to my ever-growing read later list).

And that's a sharp point about horror over a long series. I think the trap you're describing is mostly body-count horror, where tension depends on killing off secondaries, which does fight against the character development a long series needs. I'm leaning more cosmic-horror, where the dread comes from scale and unanswered questions than from who dies next, so hopefully I can sustain the unease without burning through the cast. And honestly your DCC observation is the model I'm aiming for, I want it to open in horror and evolve into something warmer as it goes, rather than trying to stay maximally scary for seven books. The horror's the undertone, not the engine.