Thank you for your answer.
In the meantime I have been convinced elsewhere to drop the idea of using collected soil, so at the moment I'm looking at a mix of sphagnum peat, sand, red clay and compost. I still have to decide what the proportions will be.
Yes, because of the little bulldozers I was already planning on using something like that to keep the layers separate, although I'm also planning a microfauna of worms and other invertebrates to live in the soil, so I don't know how long the separation will last anyway.
The pilot tank is exactly for that reason. It'll be a lot easier to set up a traditional tank for two pairs of rams coming from 120 litres in case things go downhill than for discus, tetras, plecos and corys coming from 1200 litres. That way I can learn from the smaller tank and then apply that experience on the larger one.
As for the journal, I'm planning to put the lot on YouTube and open myself to ridicule 😀
... and advice will always be welcome, of course. I've never been shy of publishing my mistakes when I was keeping a video journal of my reef, even when i had a total wipeout.