Well, I'd start with physical buttons. Forget stuff like face ID. A button that scans your fingerprint is a lot simpler to "get". Same goes for volume keys.

Automatic screen brightness is pretty good, but if it weren't a thing, buttons would work there. That's how laptops do it.

I'd add a feature that makes certain settings reset to "default" after a configurable amount of time (or never). Airplane mode or mute could turn off over night, so grandma can never "disable" her phone and become unreachable, or unable to reach anyone. (Except by turning it off, a concept almost no-one has to be taught)

Give me the ability to disable quick settings in the notifications shade, grandma doesn't need to toggle nfc, wifi, her data connection, or start screen recording (I literally tried to remove all the quick settings, but there's a minimum!). Hell, get rid of the notification shade completely and make it a physical button that just opens your messages from whatsapp, sms and email, all in one list.

I don't think we need to dumb down everything a phone can do. And I think we can assume an elderly person can get help with changing settings or setting it up to begin with. As such, what I wish fir, is for the simple stuff to be even simpler, and for the complicated stuff to be hidden away and essentially have configurable child locks, so they can't be touched, except by someone who knows what the stuff does.

It should be possibly to put a device in a mode where it is "senile-proof". But it isn't. My grandmother can, and has, put her devices in a state where they do not work, simply by turning on airplane mode without realizing. And our current solution is to use Life 360, so we can check that her phone is still "online" and have someone visit her to fix it, if it isn't.