I could think of lots of ways to split it into manageable parts. One example is to use a master comp for all the elements, and have 20x cameras. When elements go across screens you just move into that comp with linked properties and elements. That way you have a consistent screen space, but the image is broken into separate comps. Or create a master project at less than 1/4 resolution as a guide, and then scale up vector elements. And as Stephen Van Vuuren is so fond of telling us, AE simply does not have the frame buffer capabilities to work in a single 20K comp. You'd need to use Nuke
-TG