> I expect that designs for 'permanently underground/inside' cities would need to include some high-ceiling park-like areas with some bright UV lights, and other considerations, but that sort of thing seems pretty doable.

Definitely doable. But then there's no longer any special appeal to living on Mars, as opposed to: living in rotating space habitats among the asteroids.

If we reformulate Musk's goal as being: "Create off-site backups of human civilization", then I think asteroid mining & space habitats have a better shot at bootstrapping this process than colonizing Mars.

Once we are leveled up this way in resources and technology, building settlements on Mars can be a side-effect of this outcome. Just like the burgeoning scientific outposts on Antarctica are a side-effect of our current civilization.