As far as I know, the Jeux d'Anches includes the 2ft and mixture ranks. It is interesting to see if the 4ft ranks are not used yet if you should add them, but I think you should. You may want to look a bit at Franck's own organ or the organ the piece was intended for on which Anches are available there and sometimes you may want to skip a 16ft or Mixture rank.
If it says Jeux de fonds et Jeux d'anches de 8 p. it means that you only should use the 8ft reeds. During the piece you will remove anches (Otez) and later get them back (ajoutez). This is done by the lever at the bottom of your screen. As it never says that you should add more anches (change registration) for this Choral it seems most logically to just add those 8 ft anches back, by using the lever. At your taste, you may want to add some extra stops for extra effect, but it is not prescribed here. In principle, the ff and fff only indicates the swell and not the overall volume (which may want you to pull extra stops).
It is always good to check some recordings of other people to check what is done there.