On 5:4 I would only have two lanes, but I could split vertically as well so I usually still had 3-4 windows at once. On 16:9 I find splitting into 3 lanes makes them slightly too narrow, but it's workable.

As for fonts, I always make my text size as small as possible on every device I own (well, the ones primarily used by me at least).

But doesn’t this make a vertical line right in front of you, making content outside of your central vision? I like my main windows to be right in front, and less important things to both sides of it - that way, I don’t have to constantly turn my head.

Having a split under your nose feels like reading a book but putting the page you’re currently reading past your ears

Hmm.. I guess, but it doesn't matter. With dual screens there is a split right in front of me. My chair swivels so I can adjust my body to make looking left or right comfortable. I've never really thought about it and don't think it's a problem. When I read a book I don't fold it back on itself, both pages are visible at once.