Hello,
From what I understand (If I understand correctly), your processor is a
powerpc64 Big Indian, not a powerpc64 Little Indian and nowadays the
rare Linux ditros that still propose a powerpc64 portage do it for Litte
Indian (ppc64el) as Debian do. So Debian is not compatible with your
hardware.
The only distros I have found that seem to support your hardware are Gentoo
https://www.gentoo.org/downloads/ (download the PPC64 stage3 image, not
the ppc or ppc64le ones)
and Adelie (the distro is in RC (release candidate) stage but ther are
even live images)
https://www.adelielinux.org/download/
Even the *BSD seem to lack support for the Imac G5
