Well the fun fact is that you just summed up everything that I had in mind for the magicians themselves xD
So I don't really have anything to add up to it :/
Because I had the exact same idea about them :) That's why I said the thing about the secret base, only then with your idea of multiple dimensions, that sounds quite better ^^
But I think that making it a 4/10 is a bit big... Because the magicians are a race that is about to die, so perhaps it's more suitable with a 2/10?
That with every 100 humans, there are 20 magicians within them. That way the magicians truelly are a dying race, instead of still having quite a big population among the humans themselves :)