There are many fascinating views regarding material constitution. It seems to
Good question. Some philosophers would say that we really are fictions in just the sense that you suggest. In the case of our psychological being, the idea that we're fictions has a long history, going back at least to early Buddhism, featuring prominently in David Hume's thought, and continuing into the present with the views of such philosophers as Derek Parfit. The same sorts of arguments can be used to support the conclusion that composite physical objects are fictions, and in fact this idea is also very old. Over 2000 years ago, the Buddhist monk Nagasena used the analogy of a chariot, which he understood as a fiction in your sense, to illustrate the Buddhist idea of "anatta" or "no-self."
Most of us probably have the sense that not all composites are equally fictitious. Some objects embody homeostatic mechanisms that help them persist in the face of influences that would otherwise tend to destroy them. Biological systems, including humans, are the most obvious examples. Some objects illustrate what Paul Humphreys describes as a "fusion operation" a real physical operation that binds components together, introducing what is in effect a higher-level entity. An obvious example would be an atom, which is not merely a collection of sub-atomic particles. Without further argument, we can't conclude from the fact that an object is composite that it's an ontological fiction.
Whether talk of "essences" will help us much here is hard to say. To the extent that it will, one might suspect that the "essences" will be cashed out in more revealing terms. But the question of what, if anything, marks the difference between an object and a mere collection is a deep and interesting one.