Titles strike me as something that didn't matter in Europe. The Kings of England called themselves "King of France" for almost the entire duration of time between the Hundred Years' War and the end of the mod. The Roman Emperor ran Rome for a brief period at the start of the mod, then ran some land around modern Istanbul. The Holy Roman Emperor was a chap who owned random bits of land everywhere on the Continent EXCEPT Rome. The Most Christian King of France sided with the Protestants in the Thirty Years' War, then his successor promoted the divine right of kings. The Christian Monarchs of Spain authorised many thousands of killings of converts to Christianity.
Kings didn't care what other kings called themselves. The only title that mattered was "Pope", and you have already implemented that.