I don't know exactly what you mean by "advanced Japanese", but it sounds like you've hit a place I did some time ago. It seems like after a certain point, there is no formally structured way to learn Japanese as a foreigner.
Here's what I did: I started reading books and tried to apply my Japanese in life. There will always be new things to learn about the language, but after a certain point, the best way to learn more is to use it. In your usage of Japanese, you will be able to find out what you are lacking and then be able to work on that area.
I would write things in Japanese if I were you and perhaps offer them for correction, to somewhat more directly answer your question. Why not start some threads in Japanese? I'm sure our Japanese members would be more than happy to help you!