Is tourism=viewpoint the correct tagging for this? Typically a viewpoint has a view of more than just surrounding trees, a building and the sky. . .

And there certainly is no sign or other item showing this has the name of "Viewpoint at the end of the Universe".

Unfortunately, I don’t see a good existing tagging on the OSM Wiki for a location used for star parties. So I don’k know what else to suggest. I will start a conversation on the OSM help forum to see if anyone has a good suggestion.