What about Mt. Taranaki?

https://www.heywhatsthat.com/?view=AVRM3QXH

Mostly ocean, but it seems to cover a wider area.

Mt. Diablo is in Contra Costa County, not Oakland.

And at the right time of year there are tarantulas everywhere. Awesome spot.

Loads of NOPE, thank you.

Thanks for the correction. It felt a lot closer, but I haven't been there in a long time.

The way of getting the land area of the viewshed from this presentation is not immediately obvious.

My non-scientific method was to put the two windows side-by-side, and click the "zoom out" button three times to eyeball the difference.

Not to be flippant, but it's fascinating to think how an average airline passenger may trivially (and very approximately!) match or even beat such a land-based record simply by looking out the window at 40,000ft. *about twice Kilimanjaro's elevation, for the record.

Many semantics potentially apply, of course! But the principle is there. The principle that, for many decades now, humans have been matching the entire planet's largest natural viewshed, as a trivial matter of course during the everyday usage of technology, and barely even noticing most of the time...

Off the top of my head, I'd figure Mt. Shasta must have a much larger viewable area than anything in the Bay Area.