Pretty much hit the nail on the head! There are no explicit mechanics for what you get from communing with the FDM or getting infected by it.
The two could be linked (get infected, now try and commune with it to find a cure or something). I like to think of the FDM as rather, cosmic horror eldritch. What exactly is it? Why is it consuming everything in its path? Communing with it could lead you toward those answers (or something else entirely depending on how the setting is unfolding in your game).
And for the infection, I like the idea of taking a level in FRGMT. You could also create a new ability the infection grants you, but one that comes at a great cost (impending death, total takeover by the will of the FDM, spread the infection unwittingly, etc).