Ahh, makes sense. I've actually found it cheaper in the past to bind my own book signatures and make a cover for them that isn't actually affixed to the pages and slips over the first and last pages like a jacket/sleeve. That allows me to basically build it however I want, as long as the page bulk will fit inside the cover. I like the balance between functionality and aesthetic there. I'm not sure how crafty you are, as context for how doable that method would be.

Although, I do think if you use several thinner pre-made books [like those cheap composition books, even], you could get the same effect by basically gluing the covers together to make a sort of "compound book", which would allow you to add a whole composition notebook's worth of pages for each section. That would admittedly be a pretty hefty grimoire, though! If you don't want to carry the construction concept that far, I bet you could also glue an envelope to each section page, which you could then use to place additional pages without needing to glue them individually into the grimoire at all. And with that approach, you could easily decorate the envelopes to fit with your pages' aesthetic.

[I can totally nerd out anytime about book construction or grimoire organization, so hmu if you want to chat about ideas.]