Take a look at the immutable distros like Fedora Silverblue. It would install updates automatically, and has the ability to always rollback to a working version. I haven’t used it long enough to have version upgrades tested. Perhaps it asks for user input. These upgrades happen twice a year.
If I was doing that these days with my current skills, I’d install some minimal version of Arch Linux and probably would remote into it once in a while to update, or invent some simple script to do the updates unattended. The lesser the packages the easier the whole task.
Also, don’t forget there’s Chrome OS which you can install on a regular PC. (It was called Chrome OS Flex last time I did that for a relative.) It’s the easiest I can remember right now. That’s for situations when all they need is actually just a browser. For those cases Chrome OS shines.
Haven’t used one myself for years (close to a decade). Installed it for a relative about 5 years ago, never maintained it ever since.
What’s wrong with it?
It worked pretty well on an ancient PC which was running some Windows 7 if not XP. Can’t remember really. The relative is about 80 years old, so all he needs is a browser. So, Chrome OS came naturally. The hardest part was, for some really stupid reason Google wants Google account password to be entered upon booting, and not some other password. PIN code didn’t work for us for some reason. The solution I took is we changed the password to his birthday (perhaps with some A letter, if it wants at least one letter to be present). The password included dots, which was trivial to enter with a Numpad. Like A1945.09.05. But personally, I just hate it. There are use cases when you can allow a computer to have no password. Here, Google forced us to use less secure password, out of convenience. I’d prefer to have my Google account having stronger password, and forcing no password of my computer at all. The potential security risk is someone breaking into the house, and surely they’ll be very dumb to steal that computer, to have … what? YouTube history of some old fart? But that’s a bit of a different story anyway.
Me, I’d rather go with some very minimal distro and maybe even kiosk-mode browser, if necessary.
Still, what’s wrong with ChromeOS? Did I miss something important? Beyond Google dropping ‘don’t be evil’ obviously.