For grannies, I like to go with Endless OS. Curiously, it combines aspects of the two most named distros under this post; based on Debian and utilizing OSTree (like Fedora Atomic does).

It's often overlooked (for some reason), but actually combines the best of both worlds:

• Over two years of support (since release), while Fedora Atomic only offers 13 months of support since release
• Automatic updates are enabled by default and updates are applied atomically in the background, while Debian(-based) are not capable of atomic updates
• Does not even offer installing software through apt and doesn't even have it's own rpm-ostree counterpart. Instead, it goes all-in on Flatpak.

The only thing that might give something like Fedora Atomic an edge would be by installing any of the opinionated uBlue images (like Aurora/Bazzite/Bluefin etc.) that just apply and ship fixes for you (without requiring you to do anything for it) and that are even capable of automatically applying updates to major releases for you in the background. This is basically just hands-off mode.