I recognize this will vary depending on how much you self-host, so I’m curious about the range of experiences from the few self-hosted things to the many self-hosted things.
Also how might you compare it to other maintenance of your other online systems (e.g. personal computer/phone/etc.)?
Very little. I have enough redundancy through regular snapshots and offsite backups that I’m confident enough to let Watchtower auto-update most of my containers once a week - the exceptions being pihole and Home Assistant. Pihole gets very few updates anyway, and I tend to skip the mid-month Home Assistant updates so that’s just a once a month thing to check for breaking changes before pushing the button.
Meanwhile my servers’ host OSes are stable LTS distros that require very little maintenance in and of themselves.
Ultimately I like to tinker, but once I’m done tinkering I want things to just work with very little input from me.