The Bible, for better or worse, uses words differently than modern Christians. It almost always uses the term "God" for the eternal, immortal God. Even the most explicit parts of the NT are reluctant to use the term "God" for Jesus directly. Col speaks of Jesus has having the fullness of God dwelling in him. But Paul generally speaks of God as the Father of Christ, distinguishing between God and Christ. The NT uses God of Jesus in at most a couple of places, and even there it's oblique. (The clearest is probably John 20:28.)
Hence the OT statements that God is different than man are completely consistent with the way the NT uses "God."
Christians call Jesus God. This is largely non-Biblical terminology. I'm more comfortable saying that Jesus is the incarnation of God, i.e. God's way of appearing as a human. However Christians think that this human is so completely united to God that we need to think of them as one thing. There is some Biblical basis for that. But the Bible doesn't use God to refer to Christ in his humanity. The passage you quote is quite typical of Biblical language about God, even in the NT and Paul.
By the way, I'm not sure where you get the impression that the NT was written centuries after Jesus. Even skeptical scholars generally concede that Paul was the author of at least many of the letters in the NT, and that some of then were written 30-40 years after Jesus' death. It's also quite common to believe that Mark was written around 64 AD, and all or almost all of the NT would have been finished before 100. When I was in college in the 1960's it was common to hear that parts of the NT were written in the 2nd Cent (although even that isn't "hundreds of years" later). But when an actual manuscript containing part of John was dated to the early 100s, that kind of shook people up. After all, it's unlikely that what they found was the original .And John is believed to be the last of the Gospels. Similarly, as scholars started learning more of 1st Cent Judaism (partly due to the Dead Sea scrolls and other finds), it started looking like the NT documents were consistent with 1st Cent Judaism. So the dates largely moved back into the 1st Cent.
I have no idea why any Christian would consider the OT more reliable than the NT about Jesus. Yes, the prophets had hopes for the future which Jesus fulfilled. But he fulfilled them in ways that weren't foreseen beforehand. You really need to look at the NT if you want to understand Jesus.