> I think scientism runs deeper than just a set of philosophical beliefs. It is more like a modern religion, or even an aesthetic.
I agree.
> A key feature of scientism is that science itself is never well defined or understood by its adherents, so science is a floating signifier that can mean whatever its proponents want it to mean. Typically, these are not people with firsthand experience doing science, but consumers of second and thirdhand science media and science culture...
I don't think that completely true. I think you have different levels of adherents of scientism, and what you say is definitely true of most of the lower levels ("consumers of second and thirdhand science media and science culture").
However the top tier consists of the creators of a decent chunk of that "science media and science culture." Many of those people are actual scientists, but ones who have sought out the public eye as "science popularizers" and are best known for their works for general audiences.