Fonts appear fuzzy when connecting my Mac to an external monitor

You're missing something entirely. There's absolutely no requirement of having a 1.6 aspect ratio on external monitors on a Mac.

As long as you do not configure your displays to be mirrored (i.e. showing the exact same image), then the internal and external monitors are treated as separate displays. The resolution and/or aspect ratio of one monitor has no impact at all on the other monitor.

Regarding the 2 physical pixels for Retina effect, you're also missing something entirely here. An external monitor with a 1920x1080 resolution is not going to be in rendered in Retina mode. That is only for high resolution displays. Retina is not part of the equation here at all.

I would ensure that you have chosen font sizes that are large enough for the display. This should cure "fuzziness". You might also want to double-check with a different monitor to ensure that it is not this specific monitor that has a problem.