Introduction
Before addressing the question itself I will state my position on the subject of millennialism:
Whether the Millennium of Rev 20:1-6 must be literal or can be symbolic is determined by the combination of:
- the total number of the elect, and
- the time elapsed between Jesus' First and Second Comings.
Because if the total number of the elect is very large, as implied by the size of New Jerusalem in Rev 21:16 (using the Egyptian stadium, each side is 12000 x 0.1575 = 1890 km long), and the time between Jesus' Resurrection and his Parousia is short, then it is necessary for many of the elect to come to exist that the Parousia be followed by a literal Millennium during which the earth is populated by an aristocracy of resurrected saints in glorified bodies - and therefore not able to sin, die or procreate - ruling with Christ (physically or spiritually present) over a majority of people in mortal bodies who will procreate and die after long lives. The symbology is clear considering the geometric connotation of numbers, which was important for the ancient: a cubic time (10 x 10 x 10) for populating a cubic city.
Thus, whereas for Christians of the II and III centuries like St. Justin Martyr, St. Irenaeus of Lyon and St. Hippolytus, who thought that the time between Jesus' Resurrection and his Parousia would be of a few centuries (Hippolytus, the one who expected a Parousia farthest in time, expected it for 500), it was logical to understand the Millennium literally, for us, who know that said time is of at least 2000 years, it is logical to understand the Millennium symbolically. In both cases "logical" means "consistent with a large total number of the elect" [1].
The composition of the earth's population during the Millennium
I will divide the original question into 2:
Q1. Are the faithful who had part in the first resurrection (Rev 20:4-6) in a state of glorified body?
Q2. If the answer to Q1 is "Yes": Does that set of faithful include A) only the faithful killed in martyrdom during the tribulation or B) all the faithful who remained such until the end of the tribulation, i.e. both those who were martyred during the tribulation and those who got to the Parousia alive?
A1. Interpreting the first resurrection in a bodily sense (which is debatable as we will see in paragraph "First of all ..." of A2), from Rev 20:6 it can be inferred that the answer is "Yes" in two ways:
First and foremost because they cannot sin ("over these the second death has no power").
Secondly because they will live 1000 years, which excludes the state of mortal body deprived of the preternatural gift of immortality, in which man does not get to live one day (Gen 2:17), which evidently, from lifespans in the Bible, of which the maximum is 969 years (Methuselah), refers to a day for God, i.e. 1000 years (Psalm 90:4). This way by itself does not establish with certitude that those faithful are in a state of glorified body, because they could be in the original pre-fall state, enjoying the preternatural gift of immortatility. Rather, it confirms the conclusion obtained via way 1.
A2. The basis of this answer is Rev 20:4 minus the first sentence:
"and the souls of those who have been beheaded because of the
testimony of Jesus, and because of the word of God, and who did not
worship the beast, nor his image, and did not receive the mark on
their forehead and on their hand. And they lived (ezēsan) and reigned
with Christ a thousand years."
First of all, the aorist ezēsan in that text does not necessarily, or even more probably, have an inchoative sense [2]. Therefore the more probable interpretation is that those souls went on living spiritually the participation of the divine life (zōē, from which ezēsan comes), not that they came back to bodily life. Nevertheless, given that we are assuming the (pre-)millennialist interpretation, we will interpret ezēsan in a sense at once inchoative and bodily in a glorified state.
That said, it is not clear whether "those who have been beheaded" includes the second part of the set of faithful, i.e. "and who did not...".
If it includes it, i.e. if the answer is A), then the faithful who got alive to the end of the tribulation, i.e. to the Parousia, comprise the population of the earth in state of mortal body at the beginning of the Millennium, who will procreate and die at the end of long lives (around 120 years?) and so on their descendants, until at the end of the Millennium many from the last generations will rebel and the Final Judgment will come.
But ezēsan, always interpreted in a sense at once inchoative and bodily in a glorified state, can be understood further as comprising two cases:
that of dead faithful who come back to life in a state of glorified body, and
that of living faithful in a state of mortal body whose body is changed into a glorified state without going through death.
That this understanding is correct is clear from 1 Cor 15:51-52, where St. Paul states that both cases will occur simultaneously at the Parousia, in a passage in which he, retorically, places himself in the second case:
"Behold, I tell you a mystery: we will not all sleep, but we will all
be changed; in a moment, in the twinkling of an eye, in the last
trumpet, for it will sound, and the dead will be raised incorruptible,
and we will be changed;"
This is consistent with Jesus eschatological Discourse, in which it is clear that his Parousia will happen when some faithful are still alive.
Therefore the answer is B): the set of faithful who enter the Millennium in a state of glorified body includes all the faithful who remained such until the end of the tribulation, i.e. both
Complementary answer
I addressed the topic of the remaining part of the earth's population at the beginning of the Millennium, i.e. those who enter the Millennium in a state of mortal body, in another answer [3].
Note and reference
[1] Note that St. Jerome and St. Augustine, the first Fathers of the Western Church who held a symbolic Millennium, thought that the number of the elect was small.
[2] Jensen, Aaron Michael. "Bounded States, Negation Scope, and the Millennial Reign of the Saints (Revelation 20:4–5)". Novum Testamentum 62.1 (2019): 79-98. https://doi.org/10.1163/15685365-12341654
[3] https://hermeneutics.stackexchange.com/a/103634/15789