Disclaimer I am on Fedora Kinoite with soon Plasma 6 too.

Staying in an older Fedora Kinoite version will spare you from the breaking changes. Like currently 38 instead of 39. I would use ublue kinoite-main. You can disable animations, baloo etc. and have a very minimal experience. (Kinoite is way lighter than Fedora KDE and doesnt even include Gwenview, Okular or Kate)

Have a look at EndlessOS. Easy Desktop, immutable Debian base afaik. VanillaOS also has a Debian variant.

Immutable stable Distros are really needed. But I think Fedoras rpm-ostree is currently better, because it uses a git-like approach. I also think it is overcomplex and moves too slowly, so I imagine something may surpass it.

Automatic upgrades like traditional distros are not enough (they only annoy users but dont really apply them), you need really automatic ones.

Ublue has ublue-update on some editions, which is really nice. Fedora wants to implement some half baked solution I guess. If your Grandparents are always at home and on power that is no problem though (metered networks, low battery, AC connected).