“As we Orthodox see it, prayerful fidelity to the witness of Scripture, the decrees of the Ecumenical Councils, and the language of liturgical worship requires that the word “God” be reserved … for God the Father alone, the first Person of the Holy Trinity, who is said to be the Fount (pēgē) of all divinity and the uncaused Cause (aitia) of the other two Persons, the Son and the Spirit.”
Having taking note of this line in my recent paper “Disagreeing to Agree”, you asked where one would go to find Patristic support for the claim that the Father alone is the First Cause of All Things, hence “God” in the strictest sense of the term. If you’re reading Meyendorff and Lossky, you’re in good hands. Unfortunately—or fortunately, depending on one’s perspective!—there is no Thomas Aquinas or Summa Theologica in the Christian East, which means there is no formal or systematic reflection in Orthodoxy as to “whether the Name ‘Father’ is to be taken as the Name of God as such” (as Saint Thomas might put it). The quotation you noticed in Lossky’s Mystical Theology (pp. 59-60) from John of Damascus is about as focused as you will find:
“The Father derives from Himself His Being, nor does He derive a single quality from another. Rather He is Himself the beginning and cause of the existence of all things both as to their nature and mode of being. All then that the Son and the Spirit have is from the Father, even their very being…. Through the Father, that is, because of the Father’s existence, the Son and the Spirit exist” (On the Orthodox Faith, 1.8).
The Father’s “monarchy” (as it is called) is addressed primarily in the letters and sermons of the Cappadocians, but then only briefly and rather rhetorically. One must keep in mind that these saints—Gregory the Theologian (Nazianzus), Gregory of Nyssa, and Basil the Great—were homilists and pastors, not dialecticians or systematic theologians. I’m afraid you’d expend much precious time in your research if you attempted to read through entire sermons, for the pertinent passages are relatively few and far between.
Here’s a very small sampling of suggestive lines not cited by either Meyendorff or Lossky, or not at least that I’ve noticed, which may prove useful to you:
“God who is over all alone has one special mark of His own person (hypostasis), His being Father and His deriving His person from no cause; and through this mark He is peculiarly known” (Basil the Great, Letters 38.4).
In the same letter (section 44), Basil addresses the question of why Jesus says that “the Father is greater than I” (John 14:28), and his answer is that the Father is the “sole cause of the Godhead” and the “cause of the cause of all things” (that is, the Son or Logos).
“For us there is but one God, the Father, from whom are all things” (Gregory the Theologian, Orations 39.12, quoting 1 Corinthians 8:6).
“Unity having from all eternity arrived by motion at Duality found its rest in Trinity. This is what we mean by Father and Son and Holy Spirit” (Gregory the Theologian, Orations 29.2).
Of course, as you’ve seen from your brief study of this issue—and as I admitted in “Disagreeing”—the Eastern Fathers are themselves by no means entirely consistent on this point, sometimes using the word “God” in a broader sense to refer to the whole Trinity. This is because they were troubled at the prospect of becoming “subordinationists”, that is, subordinating Christ (in particular, but also the Spirit) to the Father in a way that might end up calling into question the true divinity of the Second and Third Persons.
Needless to say, they needn’t have worried. The “relative Absolute” is relative only at a metaphysical level, where the legitimate demands of bhakti do not obtain.