reader-view doesn't show the images, and for me the webdesign is too hostile to read the content without reader-mode.
Please don't build websites that are attention-expensive to consume, everything about it is terrible.
If your content is text, ship it on a neutral background, everything else is not acceptable.
Regularly happens in "book world". Chapter headings are often an entire page, and sometimes even a double spread. The average whitespace around headings on the three closest books at hand were all 1/3rd of the page, making that decor even greater than the 20% you're describing.
You can't expect everyone to reverse engineer your browser to make your reader mode experience better. Reader mode is explicitly a client side algorithm, parsing and displaying a document for you, the user. If reader mode doesn't work right for you, you should file an issue with your browser maker. After all, they're the ones that designed the algorithm to partially hide the contents of the web page for you.
For what it's worth, I have no issue viewing the images in Firefox's reader mode.
I can't see any super distracting content on this page anyway. There's a black side bar and the text is almost black on white. Perhaps you're referring to the header, but even that's mostly white and black. There are definitely some places where accessibility can be improved, but this is hardly "not acceptable".
This kind of thinking is what got us brutalism. I think I prefer the embellished versions.
The person who made this website apparently prefers something different to you and me.