CEC is attraction of positive charged particles to negatively charged CEC sites in media. Restrictions of water flow will slow it down but it will never stop until all the medias CEC potential is completely exhausted.
On a molecular level it sets up it own water exchange. As minerals are pulled out of solution and bound, the water becomes less dense and rises and is replaced by denser water. Even if you put clay substrate in bucket a couple inches deep and fill bucket with water of a known ammonium or ther positive cation element level, no circulation at all, it will continue to absorb those all on its own until all the CEC potential of clay is exhausted.
Use a large pored substrate like oilsorb or soil balls and that natural water exchange can happen very quickly. Real soil type substrate also contain humates which allow them to also absorb other negative charged anions such as phosphorus and iron. Clay, out of the package can’t, but over time it’s pore structure will absorb humates which will increase its ability to attract anions.
With you planning on putting soil over clay it will build up a humate level pretty quickly.