How to "Optimize for" an external monitor, downscale resolution but supersample for clear text?
I think you're misusing the term supersampling.


  Supersampling is a spatial anti-aliasing method, i.e. a method used to remove aliasing (jagged and pixelated edges, colloquially known as "jaggies") from images rendered in computer games or other computer programs that generate imagery.


macOS has always had support for TrueType, so any issues shouldn't be a result of poor antialiasing in any case.

What you really want to do is change the scaling of your external monitor while keeping the native resolution. You should be able to accomplish this using SwitchResX with a HiDPI profile.