iBook seems happier today. I figured out which boot settings to use to prevent the freezes; eventually I will make those settings permanent in yaboot.conf.
I also found out how upgradeable this box is even today: up to 1,25 GB RAM and SSDs up to 480 GB with the required parts available from a specialist store. The RAM upgrade will probably happen in the next few weeks; the SSDs are somewhat pricey and I'll need to think on it.
@windmills As far as I can tell, it's still used in new releases of Lubuntu. Dunno about other Ubuntu flavors; I had never come across yaboot before I switched fully to Macs a decade ago.