I also think that a Chemical element is not a substance (properly speaking).
Consider O2 (oxygen gas)and O3 (ozone). Obviously they are formed by the same type of atom, but they are different substances.
It seems to me that an Element, first and foremost, is a mental structure: an idea, not something concrete, physically real.
It is like the "collection" or "group" of atoms with the same atomic number or number of protons, rather.