Everyone on here knows I do not recommend soil substrates. The reason being that I have so far not found any substantive benefit, to compensate for "likely" problems. So although I have no direct experience, I have done my research, including Walstad's book, articles and her forum.
One thing that has to be made clear is the method. The "pure" Walstad method is one thing, but adjusting various components means you are no longer doing a "Walstad" method but something else. And as methods differ, so do results. It is obvious from the posts of other members in this thread that there are many possible means to a thriving planted tank. But one has to understand the method, and be prepared for what may occur.
If you want to set up a fish tank in one day, with fish in it, soil is not the way to go. I have done this dozens of times using plain sand or plain gravel, and never see ammonia or nitrite. This is much more difficult to do with soil, and from what I have read it is perhaps impossible (at least when dealing with an existing full tank of fish). Walstad warns of likely ammonia and sometimes at high levels, and for the first couple of months, which is why she uses so many stem plants and floaters to help take this up. Other equally-qualified authors suggest not adding fish for six months until the tank's biological system has settled. Many recommend dry starts as being safer. Redchigh should be in this discussion as he has gone down some of these roads.
Mitch, I am going to be completely frank here, as anything less to me is not helpful. With the issues you have had, which were due solely to high light, you are not going to solve them with a soil substrate. To get back to my opening comment about the "pure" Walstad method, you need to reduce you light at least by half. And if you are going to do that, why not just do it with your present setup and not add further issues?