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.
@Reinderdijkhuis Have you been looking for parts at https://macsales.com?
@starbreaker Through a different URL, apparently, but yes. I stumbled upon it through Everymac.com, which helped me identify the exact model of my iBook. It's older than I thought it was - late 2004 instead of mid-2005. This is what I can still upgrade it with: https://eshop.macsales.com/upgrades/apple-ibook-g4-1-0ghz-12-inch-opaque-white-early-2004/memory
The RAM is super affordable and I will probably order it. It's been mentioned as one of the best things I could do for it. But I could also really use a new top plate/trackpad.
@Reinderdijkhuis Getting that 120GB SSD would probably do wonders for your performance and battery life. :)
@starbreaker Well, it's been mentioned as the SECOND-best thing you can do for your old machine. But it's much more expensive than the very best thing.