I see OT God/NT God as a both/and, not an either/or. There are plenty of examples of God’s loving and patience in the OT and God’s wrath in the NT (Jesus’ clearing the temple of money changers and the plagues of Revelation, for example). We should also remember that these books were written not only by the inspiration of the Holy Spirit but by the hands of men who had vastly different experiences of God. NT writers knew a God Who had truly walked amongst them, eaten with them, wept with them, died for them.
Something else to think about, from the Catechism of the Catholic Church:
128 The Church, as early as apostolic times,104 and then constantly in her Tradition, has illuminated the unity of the divine plan in the two Testaments through typology, which discerns in God’s works of the Old Covenant prefigurations of what he accomplished in the fullness of time in the person of his incarnate Son.
129 Christians therefore read the Old Testament in the light of Christ crucified and risen. Such typological reading discloses the inexhaustible content of the Old Testament; but it must not make us forget that the Old Testament retains its own intrinsic value as Revelation reaffirmed by our Lord himself.105 Besides, the New Testament has to be read in the light of the Old. Early Christian catechesis made constant use of the Old Testament.106 As an old saying put it, the New Testament lies hidden in the Old and the Old Testament is unveiled in the New.107
130 Typology indicates the dynamic movement toward the fulfillment of the divine plan when "God [will] be everything to everyone."108 Nor do the calling of the patriarchs and the exodus from Egypt, for example, lose their own value in God’s plan, from the mere fact that they were intermediate stages.
140 The unity of the two Testaments proceeds from the unity of God’s plan and his Revelation. The Old Testament prepares for the New and the New Testament fulfills the Old; the two shed light on each other; both are true Word of God.