I see it that way.
If your project grows so big that you need to think you need to split it into different projects then I believe Sparkle is not the right tool for you to stay productive.
In that case you really need to look into a content management system, those systems are made for a large number of pages.
By saying CMS I am not referring to WordPress, that would be the worst choice.
If you really want to stick with Sparkle then I would suggest getting one of those CMS systems that you can integrate into a static website via embed Html. There are a couple of those CMS out there but not for free I am afraid.
I am not aiming to create such big websites, that’s why Sparkle is just perfect for me and satisfied how the page management, of course it needs a bit of improvement but that will come over the next versions.
I believe it needs to be addressed different things in Sparkle before the development team things on how to manage megaproject.
that is my five cents.