Was the Assumption of Mary a belief in the early church?

This article The Assumption of Mary | by Father William Saunders states that [t]he belief in the Assumption of our Blessed Mother has been longstanding in our Church, and that [i]n Munificentissimus Deus, Pope Pius XII cited various Church Fathers to trace the longstanding tradition of the belief of the Assumption.

This would be a starting point.

A similar argument to Peter's on Pentecost for the Resurrection of the LORD, can also be used i.e. absence of a grave.

Please see also Apocryphal Works on the Assumption of Mary | New Advent and The Early Church Fathers on the Assumption [Catholic/Orthodox Caucus], etc.

This discussion overlooks a few key texts.

Here is a prayer written by St. Athanasius of Alexander (296-373 AD), considered the most strident defender of orthodox Christianity against the threat of Arianism:

It becomes you to be mindful of us, as you stand near Him who granted you all graces, for you are the Mother of God and our Queen. Help us for the sake of the King, the Lord God and Master who was born of you. For this reason, you are called full of grace. Remember us, most holy Virgin, and bestow on us gifts from the riches of your graces, Virgin full of graces.

If Athanasius believed that Mary was just like any other woman when she died, what's he praying like this for?

St. Ephraim the Syrian (306-373 in Syria and Edessa) wrote sermons and hymns that were widely used throughout the Christian world. His works were translated into Greek, Armenian, Coptic, Slavonic, and other languages and are still used in many churches. St. Jerome (347-420) wrote that churches would use Ephraim's hymns after the scripture readings during their masses. Here's what Ephraim wrote in his 4th hymn on the Nativity of Christ:

Of a sudden the handmaid became the King's daughter in Thee, Thou Son of the King. Lo, the meanest in the house of David, by reason of Thee, Thou Son of David, lo, a daughter of earth hath attained unto Heaven by the Heavenly One!

In his 11th hymn on the Nativity, he wrote (titled loosely "I shall not be jealous of my son"): The Son of the Most High came and dwelt in me, and I became His Mother; and as by a second birth I brought Him forth so did He bring me forth by the second birth, because He put His Mother's garments on, she clothed her body with His glory.

Yes, both Athanasius and Ephraim wrote in the 3rd-4th centuries AD. However, it's impossible that both of them generated new dogma out of whole cloth with not a ripple in the documentation of the time. Both of them were embedded in well-known and highly-documented locations and an introduction of entirely new Mariology in those locations would have been baffling and controversial.

Athanasius' writings were widely read throughout his time and later centuries. Ephraim's hymns and sermons were translated and spread throughout the Eastern Church, and the Latin Jerome attests to churches using his hymns in liturgy.

These figures' writings, well before the Council of Chalcedon's definition of Mary as theotokos in 450 AD, show that the belief in Mary's fate after her earthly life was both widespread and well accepted. While that clearly can't say that the belief in the Assumption/Dormition of Mary was present at the beginning of Christianity, it does strongly document that it was a widespread and well-accepted belief at/near the very beginning of Christian "officialdom" within the Roman and Edessa worlds.

Pope Pius XII's 1 Nov. 1950 Apostolic Constitution defining the dogma of the Assumption of the Blessed Virgin Mother, Munificentissimus Deus, says "that, since ancient times, there have been both in the East and in the West solemn liturgical offices commemorating this privilege." He then mentions the Roman liturgy, Gallican sacramentary, and the Byzantine liturgy of the Dormition and Assumption of the Blessed Virgin Mother.

Pope Pius XII also mentions or quotes St. Anthony of Padua, St. Damascene, St. Albert the Great, St. Thomas Aquinas, St. Bonaventure, St. Robert Bellarmine, St. Alphonsus Liguori, St. Peter Canisius, and Suarez, all of whom supported the Assumption (or at least did not deny it).

St. John Damascene (675-749), Encomium in Dormitionem Dei Genetricis Semperque Virginis Mariae, Hom. II, n. 14; cf. also ibid, n. 3.:

It was fitting that she, who had kept her virginity intact in childbirth, should keep her own body free from all corruption even after death. It was fitting that she, who had carried the Creator as a child at her breast, should dwell in the divine tabernacles. It was fitting that the spouse, whom the Father had taken to himself, should live in the divine mansions. It was fitting that she, who had seen her Son upon the cross and who had thereby received into her heart the sword of sorrow which she had escaped in the act of giving birth to him, should look upon him as he sits with the Father. It was fitting that God's Mother should possess what belongs to her Son, and that she should be honored by every creature as the Mother and as the handmaid of God.

St. Germanus of Constantinople (8th century), In Sanctae Dei Genetricis Dormitionem, Sermo I.:

You are she who, as it is written, appears in beauty, and your virginal body is all holy, all chaste, entirely the dwelling place of God, so that it is henceforth completely exempt from dissolution into dust. Though still human, it is changed into the heavenly life of incorruptibility, truly living and glorious, undamaged and sharing in perfect life.

St. Anthony of Padua (b. 1195), explaining "I will glorify the place of my feet" (Is 61:13.), said (Sermones Dominicales et in Solemnitatibus, In Assumptione S. Mariae Virginis Sermo.):

you have here a clear statement that the Blessed Virgin has been assumed in her body, where was the place of the Lord's feet. Hence it is that the holy Psalmist writes: "Arise, O Lord, into your resting place: you and the ark which you have sanctified."

And that she

has risen up, since on this day the Virgin Mother has been taken up to her heavenly dwelling.

There are more quotes like these in Munificentissimus Deus.

^{See also this quote and Shoemaker's Ancient Traditions of the Virgin Mary's Dormition and Assumption.}