It's a bit like what Descartes said, I forget the exact quote, but the gist of it being some philosophers try to complicate things so much to hide or obscure the fact that they are saying either silly or trivial things. Or as Tallis cleverly pointed out in one of his books, the explanations they try to give are more difficult than the phenomena they are trying to explain.
And I think this applies to most "illusionists". It's just too obvious and when you deny things to this level, it's hard to proceed and get anywhere.
I do recall reading from you that you dislike Schopenhauer or aren't a fan. Now I can see your reasoning about it clearly. I think your reasoning is on the right track, though I very much disagree with calling Schopenhauer "stupid" - heck the fact that a good deal of the fathers of modern physics - Einstein, Schrodinger and Pauli all considered him a genius, cannot lead me to that conclusion.
But putting that aside, issues of taste are not a matter of convincing anyone, we have to attempt to look at the topic as clearly as possible. It could be that by thinking about this issue too "Kantian" or "Schopenhauerian" or even "Russellian", could be an impediment to try and clear up what we are talking about.
I agree, we have access only to representations. Even what physics tells us about the world are representations, the way we are able to discern what parts of extra mental world is made of. But we have a problem, if physics were the whole story, then we would have to posit representations "all the way down", it could be the case, but it would eventually lead to a kind of Berkeleyan idealism.
So we can say something about it, I think. Whatever the "thing in itself is", we can, more or less safely say that it is non-representational in nature, it grounds our representations, and it must be something extremely simple.
Then we can argue if it makes sense to speak of this concept as being plural or monist, or if it has in itself, any causal powers. I very much agree with you that we do not know if the world has causality as a built-in feature. Our minds appear to have such a built-in causal mechanism.
Here we enter difficult territory. So while agree with most of what you say, I depart a bit in thinking it is completely futile to attempt to give (at least) some negative characterizations of what the thing in itself could be, there are a few clues we can follow, though we will never reach certainty.