It is perfectly logical to use past and/or present titles in fantasy and science fiction stories, especially if the foreign, or past, or future language may be represented by translation into contemporary English so the contemporary English speaking audience can understand what they say.
Part One: Naval Ranks.
I note that in a space opera type story, an admiral in a space navy could have a much higher rank than any admiral in a sea navy on Earth.
Suppose that every solar system in a space opera interstellar state has a lowest rank admiral in charge of all the space warships and planetary defense systems there. Then maybe the commander of all space forces in ten solar systems would be an admiral of the second rank. Then the commander of 100 solar systems would be an admiral of the third lowest rank, the commander of 1,000 solar systems would be an admiral of the fourth lowest rank, the commander of 10,000 solar systems would be an admiral of the fifth lowest rank, the commander of 100,000 solar systems would be an admiral of the sixth lowest rank, and the commander of 1,000,000 solar systems would be an admiral of the seventh lowest rank, and so on.
"And so on"? Could an interstellar state rule more than 1,000,000 star systems? Our Milky Way Galaxy contains about 100,000 to 400,000 times as many star systems as a mere million. So a real galactic Empire ruling the Milky Way Galaxy could have 12 or 13 different ranks of admirals in its space navy.
Part Two: Civil Government Ranks.
And the same goes for ranks of civil governors. If the governor of a single star system is a stellacrat, the governor of ten star systems might be a stellacrat of Stellacrats, or a second level stellacrat, and so on, so that a galactic ruler or galacticrat might be the equivalent of a twelfth or thirteenth level stallacrat.
Suppose that a fictional galactic empire has a more or less feudal society. In that case the ruler of a single solar system might be a king. And the ruler of ten solar systems, with ten kings as his vassals, might be a king of kings or a king to the second power.
And thus the ruler of the entire galaxy might be a king of kings of kings of kings of kings of kings of kings of kings of kings of kings of kings of kings, a twelfth level king, or maybe a king of kings of kings of kings of kings of kings of kings of kings of kings of kings of kings of kings of kings, or a king to the thirteenth power.
Even on tiny little Earth, many rulers higher than kings, such as great kings, or kings of kings, or great kings of kings, have been subordinate to higher rulers such as Roman Emperors, or Caliphs, or Mughal Padishahs, or the British Emperors of India.
See my answer to this question: https://worldbuilding.stackexchange.com/questions/110223/imperial-kingdoms/110329#1103291
So it is totally inaccurate to say that there is no rank between king and emperor, or to assume that the highest ranking vassal of a fictional emperor would have to be a mere king.
Part Three: Christian religious ranks.
As for religious ranks I am only familiar with those of the Christian churches.
Christianity arose in the early Roman Empire, in the period often called the Principate.
In the early Roman Empire, most of the land was part of various city states. A civitas consisted of a city and the surrounding land that it ruled. The land of a civitas was often divided into subordinate districts called pagi, singular pagus. A Roman governor ruled a province that contained many civitates and other types of territories. The governors were officials appointed by the Emperor.
The early Christians developed parishes, areas with a single church building, often in a private home, to meet in, and headed by one or more priests. All the Christians within a city state, a civitas came to be lead by several bishops, who were reduced to one single bishop within a civitas who had authority over all the priests and other clergy.
The area ruled by a bishop, usually equivalent to a civitas, became known as a (ecclesiastical) diocese.
In time, the bishop of the metropolis or capital city of a province gained authority over the other bishops within that province, and became known as an archbishop or metropolitan. The positions of archbishop and metropolitan overlap almost totally but are slightly different in ways I don't quite understand. A province thus became an (ecclesiastical) archdiocese.
The imperial reforms of Diocletian after about AD 282 changed the structure of the Roman Empire. The governors of provinces lost all their military commands, transferred to strictly military officers, and the provinces were reduced in size and increased in numbers.
A number of provinces would be ruled by a vicar, whose territories was called a (secular) diocese, and a number of (secular) dioceses were grouped into praetorian prefectures ruled by Praetorian prefects, who the vicars were the delegates of. Thus a (secular) diocese would be two levels about a civitas, which would be a (ecclesiastical) diocese.
Thee were four praetorian prefectures in the empire, as well as four emperors under the system of Diocletian. The two senior emperors had the rank of Imperator Caesar Augustus, while the two junior emperors had the rank of Imperator Caesar.
But the Christian church was too set in its ways to change its hierarchy to match that of the empire. The only change was the gradual emergence of the patriarchs. Five archbishops gradually claimed the rank of patriarch and were gradually recognized as patriarchs with superior status over the bishops and archbishops of their patriarchies.
Since the Council of Nicaea, the bishop of Rome has been recognized as the first among patriarchs. That Council designated three bishops with this 'supra-Metropolitan' title: Rome, Alexandria, and Antioch. In the Pentarchy formulated by Justinian I (527–565), the emperor assigned as a patriarchate to the Bishop of Rome the whole of Christianized Europe (including almost all of modern Greece), except for the region of Thrace, the areas near Constantinople, and along the coast of the Black Sea. He included in this patriarchate also the western part of North Africa. The jurisdictions of the other patriarchates extended over Roman Asia, and the rest of Africa. Justinian's system was given formal ecclesiastical recognition by the Quinisext Council of 692, which the see of Rome has, however, not recognized.
The five patriarchs were the archbishops of Rome, Constantinople, Antioch, Jerusalem, and Alexandria.
And in later eras a number of other patriarchates have been created and abolished by various church organizations.
And later on, in the middle ages, the Bishops of Rome or Popes claimed to be the rightful heads of all the Christian church, and to be above patriarchs. And thus the patriarchs of Rome created several subordinate patriarchs. Today there are seven Catholic patriarchates, as well as four minor Latin patriarchates, and seven other patriarchates which once existed.
In the Western Church, a Primate is an Archbishop—or, rarely, a suffragan or exempt bishop—of a specific (mostly Metropolitan) episcopal see (called a primatial see) who has precedence over the bishoprics of one or more ecclesiastical provinces of a particular historical, political or cultural area. Historically, Primates of particular sees were granted privileges including the authority to call and preside at national synods, jurisdiction to hear appeals from metropolitan tribunals, the right to crown the sovereign of the nation, and presiding at the investiture (installation) of archbishops in their sees.2
The office is generally found only in older Catholic countries, and is now purely honorific, enjoying no effective powers under canon law—except for the Archbishop of Esztergom (Gran) in Hungary.2 Thus, e.g., the Primate of Poland holds no jurisdictional authority over other Polish bishops or their dioceses, but is durante munere a member of the standing committee of the episcopal conference and has honorary precedence among Polish bishops (e.g., in liturgical ceremonies). The Holy See has also granted Polish primates the privilege of wearing cardinal's crimson attire, except for the skullcap and biretta, even if they have not been made cardinals.2
So if the title of primate had become more powerful, it might have become an intermediate rank between archbishop and patriarch.
So in Orthodox churches the hierarchy would be bishops, archbishops or metropolitans, and patriarchs.
So in the Roman Catholic church the hierarchy would be bishops, archbishops or metropolitans, patriarchs, and the Pope.
So a archbishop or metropolitan could be considered a bishop of bishops or Bishop to the second power, a patriarch could be considered a bishop of bishops of bishops, or a bishop to the third power, and in the Roman Catholic church the Pope could be considered a Bishop of Bishops of Bishops of bishops, or a Bishop to the fourth power.
If the rank of Primate had become a hierarchical level between archbishop and patriarch in the Roman Catholic church a Primate could be could be considered a bishop of bishops of bishops, or a bishop to the third power, and a patriarch could be considered a bishop of bishops of bishops of bishops, or a bishop to the fourth power, and the Pope could be considered a Bishop of Bishops of Bishops of Bishops of Bishops, or a Bishop to the fifth power.
So possibly in a fictional interstellar space state the ecclesiastical leader of an entire planet might be a bishop to the fifth power, the ecclesiastical leader of ten star systems might be a bishop to the sixth power, the ecclesiastical leader of one hundred star systems might be a bishop to the seventh power, and so on up the hierarchy.
Or possibly the ecclesiastical leader of a planet is called a planetary primate, and the ecclesiastical leader of ten star systems is called a primate of primates, or primate to the second power, and the ecclesiastical ruler of a hundred star systems is called a primate of primates of primates, or a primate to the third power, and so on up the hierarchy.
Or possibly the ecclesiastical leader of a planet is called a planetary pope, and the ecclesiastical leader of ten star systems is called a pope of popes, or pope to the second power, and the ecclesiastical ruler of a hundred star systems is called a pope of popes of popes, or a pope to the third power, and so on up the hierarchy.
Or possibly the ecclesiastical leader of a planet is called a planetary pope, and the ecclesiastical leader of 10 star systems is called a bishop of popes, and the ecclesiastical ruler of 100 star systems is called an archbishop of popes, and the ecclesiastical leader of 1,000 star systems is called a primate of popes, and the ecclesiastical leader of 10,000 star systems is called a patriarch of popes, and the ecclesiastical leader of 100,000 star systems is called a pope of popes. and so on up the hierarchy.
Part Four: Religious ranks of non Christian religions.
Or if the religion is not connected to Christianity, maybe the religious leader in a city is called the high priest, and the religious leader of a bunch of cities is called a higher high priest, and the religious leader of a larger area is called an higher higher high priest, and the religious leader of an entire planet is called a higher, higher, higher high priest or planetary high priest or fourth level high priest. Then maybe the religious leader of 10 planets would be called four times higher high priest or a high priest of planetary high priests or a high priest of 10 stars or a fifth level high priest, and the religious leader of 100 planets would be called a five times higher high priest or a higher high priest of planetary high priests, or a high priest of 100 stars, or a sixth level high priest. And so on up the religious hierarchy to the highest high priest of all.