Alright, Story Generator is indeed a very tricky one, because even if the model would work as intended, it offers little control.
For the record, don't trust that much an LLM reply on "why things are how they are" as, for starters, an LLM doesn't think logically, it just interprets a reply based on the combination of words it faces, and more importantly, the generator itself controls how things are shown and passed, but the LLM just takes one big input and gives one big output, it is not as dynamic as you think it is.
Now, back to Story Generator, something I can advise you to try getting a better experience is to edit in the code from the Perchance side of things Line 21 which restricts to "only four paragraphs" and make it longer to ten or twenty, and also Line 45 which restrict the output to "about only about 400 words".
The reason for this, is because if the output is short, and the input is gargantuan, the LLM will have a hard time contextualizing what is going on and trying to make something "coherent" within the restrains, this is only true now since the model is still unstable, and in the future it should not be a problem, but for now it may be wise to experiment with longer outputs so the "derailing" is not abrupt.
And another thing that actually will remain true as long as the new model persists: your story as presented IS an input, so before you set instructions, you have to manually edit what you don't like, or outright prune out a whole section you find out of place. This is because your instructions and the story itself are passed together, so again, if the story is a sad dark one and you insist in the instruction "no, make it happy!", it won't happen because the model will look at the story and decide that the only "logical" step is to double down. So yeah, manual work it is. In hindsight, that gives you lee way to see the story itself as an input, as if you manually add a turning point, the LLM will latch on it and work around it instead of following a path and behavior you don't want in your characters.
Then again, I still think that Story Generator is a really tricky one to work around, I'd put it along with AI Text Adventure which even with the old model would derail into madness as soon as the second input due to how much the context would make the LLM fall into its obsessions quick. Still, with a bit of patience, all can be done, it's just that it becomes demanding and tiresome, hence why most of us don't bother anymore in trying fun long runs.
I can't promise to mod a generator for you now (I owe someone a generator, and time in my side is not nice) but I hope that with those directions you can make the Story Generator give you what you need! Best of luck!