I wouldn't disagree with anything you said. I have heard many answers to this question. What they seem to all have in common is they all involve some sort of service to our fellow man, which is cool, but to be a higher calling, shouldn't it be something higher than mankind. The answer I have settled on is.
It is the purpose of the Human Race to spread life off of the Earth. That is our Higher Calling. The primary goal of every living thing is to propagate. We are no exception. Go forth and be fruitful. Seems simple enough to me. Humans are the only animals that can get life off this rock. So that means it's our job to do so. To my way of thinking anyway.
Until that day, I will have to settle for being an Humanitarian.
Definitely, if the species is to survive, it has to move off-planet. There's no question about that. But, doesn't that still just serve mankind? Unless you think the Universe has a burning need to be populated by humans, I mean. Who knows? Maybe it does.
Unfortunately, it doesn't look like I'll be among those moving off-planet. I'll have to settle for my "higher purpose" here. (Unless someone offers me a ride on a Firefly Class ship. I own a Bowie knife and guns and my own cunning hat, and a paper umbrella, and plastic dinosaurs, so I'll fit right in.)