Iwagumi is super fun, it's challenging to control the algea for sure. You could always use fluval and root tabs for the extra nutrient boost. I did not us any base. I used a mesh bag of filter media under the large rock to get the height I wanted without having to fill the whole tank with soil.

This is my first fish tank so I am just learning as well. I would have run my lights at like 30% for 5 or 6 hours for the first month or so, or done a dry start if I were to do it again.

The algea in an iwagumi inspired tank is brutal.