I have a ten gallon emersed setup with the same lighting, same photoperiod. That is plenty of light to grow the HC well, but it will take a very long time to fill in. I started with about a 4"x4" piece a few months ago and it is covering about half of the the tank now.
I keep mine humid by keeping the top closed and putting some tape over the filter cut-out in the back. If the glass is foggy, it is humid enough.
I don't dose anything, but I'm using ADA aquasoil.
You have way too much water in there. You shouldn't be able to see the water line above the substrate. I killed a good portion of HC by having the water level too high, you only need the substrate to be wet, no standing water.
Your Rotala looks like Ludwigia repens.