https://www.macterm.net/ seems to support sixels

EDIT: And despite the option in there being on, I can't seem to get it to output graphics instead of binary garbage when I use the "lsix" bash script.

I will try to help if I can. It does work for me.

The script seems to depend on ImageMagick "convert" for most of its work; when I run "convert -version" I see "ImageMagick 7.0.6-0 Q16 x86_64 2017-06-12", is this newer than the version you are using?

I got it to work either by picking a different terminal type or restarting the app after enabling the option!

something similar exists for iTerm2