I'm certainly not saying that art and science are identical. I'm saying there's an element of arbitrariness in both of them since the concept of objectivity is overblown. Science is more objective than art, but that's partly because of a subjective aspect of art, namely the stance of objectification (link below). We objectify nature by analyzing phenomena into parts, as you say. That analysis isn't entirely in our head. I'm not arguing for solipsism or metaphysical idealism. I'm just pointing out that science is something we do, so it reflects both us and the contents of the world.
I don't know about "all art is art." That sounds like a liberal conception ("everyone gets a medal for participating"). Some artworks are better than others. And there's low art and high or "fine" art.
