Hey MaxPower42,
Have you tried nav invokers? I have not used them yet personally, but from what i understand, you basic set up a nave mesh to cover all of your landscape, but there is a checkbox somewhere in the project settings that only allows the nave mesh to be generated around nav invokers. Then you set up any AI characters that you need to move around to have nav invokers, which basic generates a small navmesh area around the pawn. This area can be adjusted based on the radius, and then it only generates the nave mesh for the radius of each character, not the whole level.
If you already know all of this, please feel free to ignore this post.
just thought it might help if you were new to it.