That did it. I threw a basic USB-A to USB-C adapter on a USB-C cable, plugged the host end into a USB-C to USB-A adapter, and got a valid tty.usbserial device created. This worked fine for connecting to the TX and for uploading new firmware.
FYI, I’m using avrdude 8.0 via homebrew. This version does not work with the suggested custom avrdude.conf. I was able to flash firmware without using that option (just dropping the -C option).