As Tom said, do NOT mess with it in Finder, you can bork everything if you don't know what you are doing.
Sorry to say I am puzzled as to how you can handle such a large project and not be aware of how to manage your media for FCP. Putting everything inside the library is ok for small projects but you need to learn about managed (i.e. all inside the library) media vs external media.
You should never have a library that large.
When there are TB of media, it should be kept in place, outside the library.
The library would be small, and all the media could be in separate SSD, one, two or three, no problem.
But once you put everything inside, you get an immense monolythic entity that needs to be all in the same place - and for which a small disk error could cause it to be corrupted and cease to work.
You should start by deleting the generated media: select the library in FCP (not the Finder!) and do
File->Delete Generated Files. Check everything.
Quit FCP. Your library should now be significantly smaller. Perhaps so small that you don't need to split it.
From here on, you could decide on a way to organize your media.
The process is not difficult but you could choose a location, and consolidate to that location, leaving stuff outside the library.
I'd strongly recommend that you learn about media management for FCP if you want to be productive and safe with such large scale productions. I suggest rippletraining, as they have excellent material.
