Good news! After much trial and error, I've ended up with a totally different, but completely functional way to get my Atari to be a dumb terminal for my Mac mini. I boot up a terminal emulator, and then use the 'screen' program to pass the shell to the Atari. This way I can take advantage of any UNIX programs I have on the Mac, which makes the Atari great for reading RSS feeds, etc. Currently using Flickerterm since Ice-T doesn't like running without an actual 850 attached, even if you load in the R: handler without it.
#Run a terminal emulator on your Atari or other vintage hardware
#Launch screen to open a serial connection between the shell host and the dumb terminal
#Use whatever your serial device is after the screen command, then the baud rate you'd like to use
#The serial device name can be found via cd /dev/ && ls tty.*
screen tty.usbserial-XXX 19200
#Once screen is launched, you can verify connectivity between the modern machine and the Atari by typing on either.
#Press Control+A then Control+: and paste the line below to mirror a shell to the Atari (or anything else!) make sure your baud setting matches after the std.XXX portion
exec ::: /usr/libexec/getty std.19200
#text input will be ddoouubblleedd on the host terminal output, but everything works normally on the Atari
#Obviously an 80 column terminal emulator is better for this
Obviously my iPhone camera doesn't like Flickerterm, the missing text is indeed present on the CRT, and surprisingly, completely readable on my LCD with the Retrotink2X! I just set it for "component" mode so the composite signal is black and white, otherwise the NTSC artifacts are a nightmare. I figured this all out from
and https://www.weinstein.org/blog/index.php/2007/06/apple-hacking-for-fun-and-profit.html