[dictum]

Apple's "elderly mode" is the iPad.

(I know it's not the same as a Mac, even if you try hard, which nobody should. However, the very things that make iOS more secure for some users are what sets it apart from macOS, making it less flexible but more resilient.)

[pugworthy]

Not a bad suggestion. Combine with a Bluetooth keyboard and he can write his long nightly emails.

The idea of "iPad mode" for a Mac is not bad. Basically it's very locked down, but has the full advantage of mouse and keyboard along with large screen.