It might have been a better choice, and maybe he didn't evaluate the option, but I still imagine the complexity of these options for the OP's specific aesthetic taste is quite large.
Which is one reason to turn to the Natural Earth data, it has been curated more systematically than OSM.
There has been lots of research about how to automatically produce Goldilocks Maps (enough information - but not too much) - I remember an article (whose URL I can't locate) which showed increased readability by pruning minor location names when they are too close to major locations. Google does that sort of thing well, but they are not the only ones nowadays - for example the osmfr Openstreetmap style does, in my opinion, a decent job of prioritizing labels: http://tile.openstreetmap.fr/
As i noticed the other day, they certainly have a very precise notion of where Great Britain is:
example image used in the article shows most countries with their capitals, and others with some random cities highlighted, truly to the T!
I do agree with others though that it would have been a great amount of effort to invest in understanding CartoCSS for his requirements, and getting to create it.
