AnchorCMS is pretty much WordPress without the bloat, while Grav is my favorite i think something that is similiar to WordPress is better and Anchor is pretty much inspired by WordPress just in modern also its just 331kb.
" I don't see WordPress as ideal as he won't be publishing new posts or pages often. We don't want to go with Wix or similar website builder."
So, you want to code it yourself?
No?
Then you're back to using a CMS or similar.
Nothing wrong with using WP as the platform for this.
Alternatively you could try ClassicPress, Ghost or even Anchor.
What I'm saying is, a CMS (whichever flavour) would be the right answer for you.
or anchorCMS which is pretty much all you'll need for a simple blog, also uses some similiar patterns as WordPress but without the bloat. https://anchorcms.com/
But WordPress will have more references/guids etc.