John of Damascus says "of the Greek we have the distinction of hypostases.". That is adding Greek doctrine to "Christianity."
When John says "we have" something "from" the Greeks he is speaking of the source of the belief, and not a mere agreement.
Is there an exegetical reason to doubt based on these two statements from Gregory and John that: the Trinity was looked upon by the Fathers themselves as a combination of Jewish monotheism and pagan polytheism?
Church Fathers attribute the Trinity to a mixture of Pagan and Jewish teachings.
Gregory of Nyssa - Oratio Catechetica 3 PG 45, 17 D-20 A
Gregory of Nyssa, Last of the great Cappadocians and brother of Basil of Caesarea, was bishop of Nyssa in 372. Gregory states that "Orthodox" doctrine is a combination or syncretizatation of Jewish monotheism and Pagan polytheism:
"the truth passes in the mean between these two conceptions, destroying each heresy, and yet, accepting what is useful to it from each. The Jewish dogma is destroyed by the acceptance of the Word and by the belief in the Spirit, while the polytheistic error of the Greek school is made to vanish by the unity of the nature abrogating this imagination of plurality."
John of Damascus - De Fide Orth. I, 7 PG 94, 808 A
John of Damascus, who followed Gregory of Nazianzus agrees that "Christianity" takes what s best in Judaism and paganism:
"On the one hand, of the Jewish idea we have the unity of God's nature, and, on the other, of the Greek, we have the distinction of hypostases, and that only."
Patristics scholar Wolfson says:
evidently the opposition of orthodoxy to Arianism was not so much on the ground that it was a combination of Jewish monotheism and heathen polytheism as on the ground that the combination was not to its liking. In fact, its own conception of the Trinity was looked upon by the Fathers themselves as a combination of Jewish monotheism and pagan polytheism, except that to them this combination was a good combination; in fact, it was to them an ideal combination of what is best in Jewish monotheism and of what is best in pagan polytheism**, and consequently they gloried in it and pointed to it as evidence of the truth of their belief. We have on this the testimony of Gregory of Nyssa - one of the great figures in the history of the philosophic formulation of the doctrine of the Trinity - and his words are repeated by John of Damascus the last of the Church Fathers.
(The Philosophy of the Church Fathers: Faith, Trinity, Incarnation Third Edition, Revised https://www.amazon.com/dp/B002MS4UN6/ref=cm_sw_r_cp_api_i_jxjkEbZW3TCV8)