4 other things come to mind:
a) the 'ud' alias isn't valid
b) 'ud' points to a wrong device (test with dev /aliases .properties) or so
c) you've got more than 1 usb stick plugged in
d) stick still somehow isn't readable by the OF, maybe your G5 has some very early OF version that doesn't handle FAT32

I know you say you've formatted it on Sonoma... Did you use commandline or Disk Utility? DI *may* do a bad job. You could format the stick in HFS+ instead. OpenFirmware will read that for sure. Run a Terminal on macOS with the stick plugged in. Do a 'diskutil list' and figure out which /dev/disk* is your USB stick. Then do 'diskutil eraseDisk HFS+ USBstick APM /dev/disk*' replacing * with the right number (do check that twice before you hit enter ;) Copy the iso and boot.img to the usb stick and try again.

Btw there's some reference of OpenFirmware commands here https://library.morph.zone/Open_Firmware