I believe role=“img” tells it to behave like one, causing the descriptive text to be read out in its place.

Yeah, that seems bad. The whole point of the diagram was to explain something better than prose could, and now it is lost. I'm thinking the case were someone can make out the shapes/arrows/colors well, but not the text.

In my case, that's not why I'm using the diagram. It's simply a visual thing, to break up a page of text. Purely aesthetic.

The aria-label would be used to describe the content.