I think this is it: the book may finally be ready for publishing. I received an author’s proof, and remembered that Amazon tends to print a little dark. I punched up the brightness of the cover image, which is a slight shame, as some subtleties in the details get lost this way. After a bit of thought, I decided that the cover is still slightly plain. I wanted it stark, but modern book covers are very (ridiculously?) busy. Thus, I added in some nuances, particularly to the lettering. I think this also helps the main image to compete with the title block a little.
Here’s the original (below):

And here’s the updated version (below):

Now that I’ve added those details, the original looks a bit plain.
Lastly, I decided I used the word “said” a little too much. The word tends to become invisible to the reader, unless it’s overused, at which point it becomes noticeable (and clunky) again. I did a pass of paring down its usage, as well as some other very minor tweaks.
If the latest proof comes back good, I literally can’t think of anything else I could possibly do to improve this book now, and it will be time to publish.
Wish me luck.