I've read numerous threads on doing it with USB Tethering, and I feel confident I could do that. However, I need it for ethernet. I can use Magisk + shell script at /data/adb/service.d. Any other scripts inserted in there I can do, but I do not know what the command is. Not too many people seem to have this question, as it's a recently-introduced feature of Android 11. Phone is a Pixel 3A with the latest PixelExperience Plus (Android 12). Ethernet tethering device is ASIX-based USB-C w/PD passthrough power. The other end of the ethernet connection goes to the router WAN port.
For how to make a Shell script at boot: See: https://stackoverflow.com/questions/7873615/how-to-run-script-at-boot-time