Debian has dropped PPC support moving forward, so you're on borrowed time. You may want to look into OpenBSD or NetBSD as a backup plan if you intend to keep that hardware long-term. I've found OpenBSD to be nearly painless to maintain and not too much of a learning curve (admittedly coming from Slackware being the Linux distro I cut my teeth on nearly 20 years ago).

That said, I don't know how difficult it would be to install a non-Linux OS on that particular device; I know it was designed for and shipped with Debian. I've had success installing the various BSDs on all kinds of obscure hardware but I've yet to get my hands on a MyBook Live.