Hi Amigaharry2!
Thanks for all your advice, I really appreciate it!

I tried setting
boot-device and
boot-command as you suggested, but still no luck: the G5 chimes, then a black screen until I switch it off by pressing the power button.
There's a difference with yesterday, though. When
boot-device is "hd:,\\:tbxi" (its default value), the G5 would show a gray screen, but with "hd:,\boot.img", it's a black screen!
https://www.reddit.com/r/ProgrammerHumor/comments/tgogft/sometimes_progress_looks_like_failure/
I put the outputs of
printenv and
boot (from inside OF)
here and
here.
Thanks!