Waldstad is something you’ll probably be interested in due to your background in ecology.
However, for the types of plants you have, anubius and moss, they don’t need a lot of nutrition from the roots. Amazon Swords are an example of a heavy root feeder that would enjoy a nutrient rich soil and root tabs. But if you’re going to tie your Anubius and moss to driftwood, then they would primarily be getting their nutrients from the water column. Also, these slow growing plants can’t uptake extra nutrients from the soil or water column fast enough. Instead, faster growing algae will outcompete and take over.
Eco Complete is inert but that’s a good thing if you don’t need to change your water chemistry. However, it is still a high CEC material that has lots of minerals and elements necessary for plants. It just has far less organics and because it’s inert, you have the flexibility to dose fertilizers as needed, which in your case, is not very much.
But this is all moot, since the type of substrate, whether you dose the water column with fertilizers, use root tabs, run CO2, or talk about different types of lighting all depends on the types of plants you’re trying to grow.
If you were wanting to raise Taiwan Bees that need softer water and will have a lush carpet of utricularia gramnifolia and a bunch of fast growing stems plants, then this would be a totally different conversation.
In your current setup with just simple cherry shrimp, moss, and anubias, if you use soil or Aquasoil and dose ferts/root tabs, you will be overrun with algae because the system will be out of balance