According to the materials, the main fundamental distinction between the Son of God and God Himself does not lie in any difference in their divine essence (which is essentially identical), but in their hypostatic difference, expressed through the relationship of generation.

The Divine Contrast: Begotten vs. Unbegotten

According to the materials, the main fundamental distinction between the Son of God and God Himself does not lie in any difference in their divine essence (which is essentially identical), but in their hypostatic difference, expressed through the relationship of generation. That is, God the Father is characterized by His unbegottenness, eternality, and self-existence, whereas the Son is defined precisely by His birth, by His genesis.

As explained in one of the sources:
"But there remains an irreducible difference related, so to speak, to the metaphysical position of each hypostasis. The Father's attribute is His unbegottenness ... The Son's personal attribute, on the contrary, is to be born— the beloved Son: 'The Father loves the Son and reveals everything in Him that He creates'... God the Father and God the Son are completely equal and united in the divine nature; in this respect, both the Father as the true God and the Son as the true God are one and the same, yet it does not negate their hypostatic difference, which is generation and begottenness." (source: link txt)

Furthermore, another source emphasizes this idea by noting that it is specifically the act of being born that differentiates the Son from the Father:
"But being spirit, the Father begot the Son without beginning and without age from Himself, similar to Him in all respects and equal to Him..." (source: link txt)

Thus, the fundamental distinction is that the Son of God is begotten, while God the Father is unbegotten. This does not diminish their co-eternal and perfect divine nature, but rather underscores the eternal relationship and the difference in the manner of their origin.

Supporting citation(s):
"But there remains an irreducible difference related, so to speak, to the metaphysical position of each hypostasis. The Father's attribute is His unbegottenness ... The Son's personal attribute, on the contrary, is to be born..." (source: link txt)

"But being spirit, the Father begot the Son without beginning and without age from Himself, similar to Him in all respects and equal to Him..." (source: link txt)