AFAIK CachyOS still demands a little involvement in the OS. Like, you have to watch the logs when you update, you need keep context in mind, like knowing you're running KDE and an Nvidia card and so on. But I feel like Bazzite would be more usable to someone who doesn't know (and doesn't need to know) what a filesystem or a discrete GPU are.
But in terms of stability, CachyOS has been rock solid for me. The cadence that Arch + CachyOS devs fix stuff has been utterly perfect.
So I say if your dad is more 'software curious,' give him CachyOS. If he doesn't like messing with computer stuff, give him Bazzite.