The wiki page "Installing OpenWrt via TFTP" has a section titled "tftpd server on Mac OS X Lion". Is there a reason it doesn't live under "Setting up TFTP server/On macOS", besides not being verified to work on recent versions?
I can verify that the built-in tftpd server does work on the latest macOS 12 Monterey, as I've just helped a friend to flash an Archer C6 v2 (EU) with their Intel-based MacBook & an USB-C ethernet dongle. A switch wasn't needed, but we did need to go slightly off the script and copy
/Library/LaunchDaemons/ to be able to edit it. The firewall was also disabled during the flashing for good measure.