First,  what is your main ...

First,  what is your main goal for the tertiary wetland, to only remove P from the secondary effluent or also to have some denitrification? If P is the case, I am not sure a horizontal flow wetland (suppose subsurface) can indeed be an option. If P is the target pollutant, it will be cheaper however to simply add eg ferric chloride in a small tank to precipitate P. The use of shale gravel in the wetland will indeed remove some P, but after a short period of time the material will get saturated and the P removal will start again deteriorating.

What I usually do to improve P in my constructed wetland designs (if I want to avoid chemicals) is (a) install a simple gravity filter with a special media (specific to P removal, eg steel slag, bauxite etc) to polish the effluent, so that I can have easy access and easily replace the material when it gets saturated. However, although effective, this is not a very cheap option and involves some additional operation tasks, (b) I go for a combination of subsurface flow + surface flow wetland (which is cheaper to build) and exploit biological processes (eg plant uptake) and filtration/precipitation, so that I can also improve the quality in general (eg nitrate removal). Of course, the level of inflow P and the desired P effluent limit you want to achieve play a crucial role in the selection of the most appropriate solution, as well as the available carbon source, should denitrification also be desired.