Hmm, so some of this sounds like I may misunderstand the terminology of academic philosophy. I'm trying to learn it, but I generally lack a lot of context for how the terminology is used so I largely have to go with what I find to be the definitions suggested by summary articles as I find I want to talk about some subject. In many cases I feel like the terminology is accidentally ignoring parts of theory space I'd like to point to, though I'm not sure if that's because I'm confused or academic philosophy is confused. Yet it seems to be the primary shared language I have available for talking about these subjects other than going "full-Heidegger" and being deliberately subtle to hide my meaning from all who would not bother to do the work to think my thoughts.

On some particular points:

I agree that we only know through experience, but your reference to how this cashes out in physical terms suggests that we might mean something different by knowing through experience. That is, I do not disagree that in fact this is how it cashed out. But the fact that it does, is a fact that we learned by experience, and from the point of view that we had before those experiences, it could have cashed out quite differently.

Sure, I only included the physical explanation because I wanted to be clear that I'm talking about a fundamental kind of thing here by "experience" and not, say, the common use of the word "experience". Unfortunately existing phenomenology lacks, from what I can tell, a rigorous way of talking about experience as generic information transfer.

I think there is conclusive proof of such a structure: some things are not other things (take any example you like: my desk is not my chair), and this is a fact that does not in any way depend on me. If it depended on me, then we could say this is my way of knowing, not the structure of the world, and essentialism might turn out to be false. But as it is, it does not depend on me, and this proves that the world has a structure on which its existence logically depends.

This is one such case where maybe the terminology fails me. Perhaps the existentialist/essentialist divide is not the one I mean. I want to separate those theories that conflate ontology, especially teleological aspects of ontology, with metaphysics from those that view them as separate. Once we have them separate, then we seem to be able to talk about idealism and realism from a perspective of structure creates reality or reality creates structure (i.e. ontology determines metaphysics or metaphysics determines ontology). It is this latter latter case I mean to be in: ontology, which is necessarily discovered only through experience) is the lens through which we can try to discover metaphysics, but metaphysics is ultimately about the stuff that exists prior to the understanding of its structure, and that there is literally nothing you can say about reality except through the lens of ontology because you have no other way to know the world and make sense of the experience of it.

In fact, overall you seem to me to be asserting a position like that of Parmenides

I'd say Parmenides has the same flavor as me, although I'd have to do some heavy interpretation to make what evidence we have of his position fit mine.

I can't draw a conclusion about this myself from what you have said: perhaps you could compare yourself e.g. statements about ethics and statements about money, which are clearly intersubjective. I find it hard to imagine someone who is really and truly non-realist about money: that is, who believes that when he says, "I have 50 dollars in my wallet," the statement is strictly speaking false, because he actually has just a few pieces of paper in his wallet, and much less than 50. But perhaps this is no different from the fact that it is hard to accept that people who claim to be moral non-realists, actually are so.

I'd say there's nothing so special about talking about ethics versus money other than they have differences in meaning and purpose for us, i.e. teleological differences. There is a useful sense in which I can say "I have 50 dollars in my wallet" or "murder is bad" but this is also all understood through multiple layers of structure heaped on top of reality that, without interpretation via experience, would have no meaning. Perhaps "truth" has a broader meaning than I think in academic philosophy, but it seems to me if we're talking about ways of experiencing the experience of reality then we've left the realm of what most people seem to mean by the word "truth". But perhaps this is a definitional dispute?