What would be the best desktop environment(?) for a very old person with almost zero experience with computers?

I've seen this question asked before, but most answers seem to assume that the elderly person in question has had some experience with Windows at the very least.
I am talking about a person who, up until a couple of months ago, literally never typed on a keyboard or touched a computer mouse.

I installed Linux Mint Cinnamon for him. Tried to simplify it as best I could: Removed a lot of the bells & whistles, increased font size, cleared out the menu to the point it only includes Firefox and shortcuts to the websites he intends to browse.
Did not even bother with desktop shortcuts because he struggles with double-clicking icons (yes, that's the level of tech illiteracy we're dealing with).

He eventually figured out how to operate things, but it's clearly still a struggle for him.
He's bit more comfortable with his smartphone, though, so I was wondering is there some sort of ultra-minimalist GUI shell to simplify things to the level of something like Android?
(and no, Android-x86 is not an option, unfortunately)

I looked into Sugar, and looks like something in the right direction, but its focus is all on like, educating Third World kids or whatever.

Thanks