You might want to read my post in the thread: Imperial Kingdoms?1
Some of the historical examples it gives from Earth history may be informative for your purposes.
To decide the title of the ruler of 6 kingdoms consider the organization of a feudal kingdom in Europe, which is more or less a rough model for your society.
Most of the land in the kingdom would be divided into hundreds, or thousands, or tens of thousands of manors, each with tens or hundreds of peasants, both serf and free. Each manor would have a lord who held it as a fief from a higher lord and had to give feudal service to the higher lord.
A larger region of the kingdom would be a county, and the count of the county was usually the overlord of most or all of the manor lords in the county. The lords were the vassals of the count. Thus the count could be considered to be a lord of lords but used the separate title of count instead.
The kingdom would be divided into some large regions called duchies, each containing a number of counties. The duke of the duchy would be the overlord of most or all of the counts in the duchy.
A duke could be considered a lord of lords of lords, or a count of counts, but used the separate title of duke instead.
All of the kingdom would be under the king, and all of the dukes would be vassals of the king.
So a count could be considered a lord of lords or lord to the second power, a duke could be considered a lord of lords of lords, or lord to the third power, and the king could be considered a lord of lords of lords of lords, or lord to the fourth power.
A duke could be considered a count of counts, or a count to the second power, and a king could be considered a count of counts of counts, or a count to the third power.
And a king could be considered a duke of dukes, or a duke to the second power.
In real life the feudal system, or lack of system, could b get a lot more complicated. Many persons accumulated a number of different lordships, counties, duchies, and even kingdoms and thus had several different overlords and many different vassals. Few persons ever turned down a chance to acquire another manor, fief, or principality merely because it would make their feudal relationships more complicated.
It was perfectly possible for Lord A to hold fief B from Lord C while Lord C held fief D from Lord A, making them both each other's overlords and vassals.
A baron was a lord who was a direct vassal of the king and wasn't a vassal of any count or duke.
In the Holy Roman Empire the class of princes were rulers who were immediate and direct vassals of the Emperor and king, but they had a number of different titles. From lowest to highest the titles of the princes were princely count, landgrave, margrave, count palatine, prince, duke, grand duke, and archduke.
In some kingdoms, abbots of monasteries, bishops, and archbishops were also feudal vassals.
Anyway, in a feudal kingdom, each higher level could have been described as the overlord of the next lower rank but instead had a totally separate title.
So that would make it logical for you to invent a totally new title for the overlord of kings, since counts, dukes, and kings had titles which didn't specify who they were the overlords of.
As most people should know, there was a country in Europe which had many rulers who were overlords of kings - Ireland.
Medieval Ireland had tens of states or kingdoms called Tuatha - I have seen numbers like 90 or 150 but don't know how many there actually were, so I assume that there were about 50 to 200.
The island of Ireland has an area of 84,421 square kilometers or 32,595 square miles. So if at any one time there were 50 to 200 Tuatha in Ireland, the average Tuath would have an area of 422.105 to 1,688.42 square kilometers, or 162.975 to 651.9 square miles. If each was perfectly square it would be 20.545 to 41.090 kilometers on a side or 12.766 to 25.532 miles on a side.
If medieval Ireland had a population of 1,000,00 persons at any specific time the average Tuath would have a population of 5,000 to 20,000 persons at that time. Of course Tuatha varied a lot in size and population.
But if the kingdoms in your story are no larger than Irish Tuatha the six kingdoms would occupy only a rather small island instead of an entire continent.
The monarch of each tuath was a ri, or king. The ruler of a group of tuatha was often called an overking. The ruler of all the tuatha within a province of Ireland was often called a king of overkings.
"Province" is an English word. The Irish word means "fifth", and the "provinces" were originally fifths of Ireland. But ambition was more important than math or logic for Irish kings, so the number of "fifths" at any one time varied with political and military events, and today people usually talk about the four provinces or "fifths" of Ireland, forgetting there were once five and sometimes six.
And for centuries there was a king claiming the over lordship of all Ireland, who was called the High King of Ireland, or often called the King of Tara from the ancient ceremonial center at Tara.
As I said before, you are free to make up any name you wish for the supreme ruler above the six kings. If the name of your island or continent is, for example, Ecalpemos, you could call the supreme ruler the Ecalpemcrat, "ruler of Ecalpemos". Or maybe Contincrat, "continent ruler".
However, I kind of prefer that you call the supreme ruler of the continent the king of kings of that continent.
And that is because a number of religious persons claim that the title of King of Kings is reserved for God almighty, and I think that silly opinion should be disregarded as often as possible.
In the Bible, God is often called the Lord, and King, so if using titles that have been used for God is bad, all the lords and kings in medieval and modern Europe would have also been using titles reserved for God.
Furthermore, when the Bible was written including sections describing God as King of Kings and Lord of Lords, human rulers had already been using the title of king of kings for centuries or millennia. Clearly the writers who described God as king of kings couldn't hope to persuade human kings of kings to give up that title, or convince the more educated Jews that king of kings was a title used only by God, so those could not have been their intentions.
And of course many Christian countries have been ruled by kings of Kings.
But claims that mere mortals shouldn't use the title of king of kings may have discouraged its use in many medieval lands. In Ireland, for example, it would have been logical for an overking to use the title of king of kings, a king of overkings to use the title of king of kings of kings, and the High King of Ireland to use the title of King of Kings of Kings of Kings of Ireland.
European culture lacks any title higher than king and lower than emperor, and so many rulers who wanted use higher titles than king took the title of emperor. Thus there were a bunch of so called "empires" such as the Russian Empire, the First and Second French Empires, the Austrian Empire, the First and Second Haitian Empires, the First and Second Mexican Empires, the Brazilian Empire, the German Empire, the Empire of India (the British Raj), the (Italian) Empire of Ethiopia, and the Central African Empire.
But none of those "empires" were true empires in the sense of being Roman Empires or claiming to be the rightful governments of the whole world. If European culture had a tradition of titles ranking between king and emperor, such as great king, high king, king of kings, etc., etc., those titles could have been used by European rulers higher than ordinary kings but lower than emperors.
And I hope that fantasy writers do their part to show what an error that was by creating titles higher than king for their fantasy world while restricting the title of Emperor to those rulers who claim to be the rightful rulers of all the world or all the universe.