Personally, while the sounds and the post-process overlay are great effects when getting towards low sanity, I feel adding more effects to keep the player on edge would be a great addition before the rotting stalkers start generating. Mainly adding non-intractable hallucinations as well as potential jumpscares (which could also impact sanity when they occur, as a snowballing effect) would allow for more interaction with the mechanic before reaching the "enemies are going to kill you" stage.