I use Stylus which let's me inject custom CSS. I used it on his site to center the content.

I wonder why reader mode doesn't work here.

used to be bothered by the same thing but have since adopted one of those "quickly position window" utilities -- so e.g. you can quickly move the whole window to a reasonable column-width in the middle of the screen

i use "rectangle" for the mac, but there are lots of alternatives on lots of platforms. very nice QOL improvement.

My browser never exceeds 40-50% of my total screen width in my tiling window manager. Keeps my eyes from shooting all over the place. For websites that are properly optimized, you don't really lose out on anything. And for the websites that aren't, a quick keybind to go fullscreen for a bit is nice.