Could we perhaps have a Magicians' Court, where there is an assembly of 3 Magicians that work for the country? If one dies, they are reincarnated into an infant of whatever race and nation that the late Magician belonged to?
Therefore, you can kill the Magicians, and they would be reborn, and would have all the wisdom and knowledge of past lives before them. However, they would have to grow up again and only begin magic at age 16.
I think this would serve as a good, balanced system.
Also, the Magicians cannot be the ruler of a country, because that's pretty op if you had an immortal king.