This exciting discussion is leading me to the insight that defining God as “self-existent” may be misleading.
In any case it is crucial to realize that:
God is NOT an “existing self” or “a single personal identity”.
God is:
“Three persons (Father, Son, and Holy Spirit) co-existing and sharing a single act of being”.
Each of these three persons is in relationship with the two others.
God’s act of being or God’s life consists in this relationship, God is a relational entity.
The Father cannot be said to be “self-existent”, in the sense of “existing by himself”, independently of the other two divine persons. And the same holds for the Son and the Holy Spirit.
The Father, the Son, and the Holy spirit are CO-EXISTENT; it is these three persons all together who say: “I am who I am”.
A further important point is that these three divine persons decided to bring about other persons (the angels and the human beings) and ordered and called them to be in loving relationship with them (the three divine persons), and with each other (the other angels and humans). To be a person does not mean to be called to exist as an individual for himself, it means to be called to co-exist with other persons.
In this sense, the human beings are projected towards unification with the three divine Persons, towards divinization. After the last judgement we will NOT become “self-existent”: We will CO-EXIST for ever with the Father, the Son and the Holy Spirit. And this is what the Fathers and Doctors of the Church (St. Irenaeus, St. Athanasius, St. Augustine, St. Thomas Aquinas) mean by stating that “God became human so that we might become God”.
At the end of the day of judgement, the creation of the angels and human beings will appear as sort of “God’s growth” that increases the number of ‘selves’ (not of persons) co-existing in God, according to John 17:21-23:
“ that all of them may be one, Father, just as you are in me and I am in you. May they also be in us […]. 22 I have given them the glory that you gave me, that they may be one as we are one— 23 I in them and you in me—so that they may be brought to complete unity.
This “expansion of God” will be achieved after the last judgement.