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.