I'm not sure I aggree with your layout?
In terms of readability, I've found people tend to ignore anything on the right-hand side. Hot spots/eye scanning software shows most people scan a page in a rough F shape. They then wander down the page. This article has a couple of good illustrations of how readers scan a page:
https://www.smashingmagazine.com/2008/01/10-principles-of-effective-web-design/
Unless your left-hand column is actively doing something (is it representing a different page in the whole? Is it a site map?) and showing you where you are 'at' in the process (like a bookmark)? Without some sort of function it seems unnecessary.
In terms of code: I'd wonder about responsive design (this could be 2 flex box columns), which on mobile will fall into one line. On that point is the right-hand column links to the other pages? Should it go first or last? I'm an educational designer at an Australian university and find that a lot of teachers don't code, or know HTML and want something that requires minimal touch.
If you search in Commons for 'Sample layout page - three evenly spaced column code' you'll find a 3 column flex book example that might interest you. There is also a 2 column version under the name 'Sample layout page - two unevenly spaced column code' that will have the flex code for the layout above.
Thanks for bringing this up - page design is the mythical white unicorn!
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