Amazon has been working of as solution for this problem. The latest version of the Kindle Previewer supports the CSS property "float: snap-block". This allows text to be repositioned around an image in a way that avoids extra blank space in most cases. It only works for Enhanced Typesetting (KFX format).
I haven't worked out all of the rules for using it and there is no actual documentation from Amazon. You can read a bit more in this thread:
Kindle Previewer 3.40 - 64bit, SVG text, snap-block.
As a test I modified your sample book to add snap-block to the first image. After some trial and error I was able to get it to work. The attached images show how the Kindle Previewer renders it at two different font sizes.