Well, that didn't work. It eventually stopped booting. Which is weird, because it was booting properly at first, but soon needed boot: old, and on top of all of that, my Powermac7,3 never had sound. It definitely thought it did, but I couldn't get it to actually produce sound. Which is weird, because at least Lubuntu had sound. You'd think Debian would too.And, while I was at it, I found some instructions on how to remove a new kernel if you got it installed. It needs a little bit of tweaking for our specific kernel versions, but I did get the new 5.4 kernel installed when I updated, and I did also use this to get it off. So now I not longer need to boot with the "old" command at boot: line.
Annoyingly, it means that "apt-mark hold linux-image-4.15.0-2-powerpc64" didn't work for me, as it said I had already done it once I tried it again, but it looks like I have a stable working Debian 10 ppc system running quite nicely.