Very nice tank!
Maybe one or two of the smaller/medium lillies would do? You can just trim the larger leaves if they get too big..
Duckweed will also work, but once it sets in, you will be dumping it weekly lol
Also, your swords will get tall and eventually provide more shade......
I also like the lights alot....
I went from 14k to 6500k, let us know how the 8k does.
You can always try some moss elsewhere in the tank and see if it grows well for you before you put together the wall.
Best of luck.