Honestly, there isn’t much to it when setting up Linux for elderly people - in fact, I find it less troublesome than setting it up for a teenager.
Most often the issues regular users face with Linux are related to installing packages from external sources or broken updates. Elderly people tend to not do that.
Set up a stable distro like Debian, Linux Mint or Ubuntu LTS with KDE Plasma or Cinnamon, install LibreOffice, Okular and a browser with strong ad blocking, and any other applications you think they might need. Enable a simple firewall, hide the root / folder from the file browser’s sidebar, and you’re done. Perhaps set up scaling to make everything bigger on their monitors, disable mouse acceleration and set the speed slightly slower than usual.
I wouldn’t bother with immutable distros, Flatpaks are nice and all until permissions turn using a simple app a confusing chore with broken interactions.
Yep! Okular is amazing, and it’s available on Windows too. Install it for someone and they’ll never bother you again about PDFs or EPUB documents, it’s performant and everything works: printing, resizing, selecting text, searching, signing, adding comments. Never worry about paid PDF software or shady slow apps that keep trying to gatekeep features.