As a personal successful experiment: NixOS/Xfce o Gnome Shell (require a bit of extra work due to not really smart basic design and extensions fragility), basically the system read-only, home on zfs with snapshots ensure enough local data safety against accidental deletion/bad change. Keeping it up to date is just only updating the config when needed and rebuild.

No commercial software so far offer anything as stable, no classic distro/OS can offer something as stable.

About phones: a Motorola GSM desktop phone for some, a Punkt for those who accept&like a mobile device that's apt for seniors while not crappy as most "for seniors" crap.