Not that it really matters, but trying to learn about (Christian) God by reading the Old Testament is like trying to perform maintenance on your 2024 vehicle using a manual from the 2000 version of that car.. Like, yeah, that was relevant once, and there's some overlap, but the situation has evolved since then. It's called the Old Testament because it is based on something outdated (again, from a generically Christian perspective). The Old Covenant (which is what the Old Testament is testifying to) was between God and the Jews, and was based around compliance with the law. That's why the OT is so full of rules and punishments.

Then, Jesus arrives on the scene and changed the game. His birth, betrayal, and death, represent a new contract between God and humanity (not just the Jews) wherein mankind is saved by God's grace alone. In fact, God has done a 180 on the whole obedience to the law thing. Turns out, God loves sinners, and prostitutes, and tax collectors, and prodigal sons, and all sorts of ne'er-do-wells that the God of the Old Testament would have reviled. From the death of Jesus forward (and maybe retroactively too, I don't know dogma all that well), the only thing necessary for your salvation is God's grace, and that is given to all, as long as you accept God into your heart or something like that. Basically, God is Darth Vader, and he has altered the deal, pray he does not alter it further.

Of course, as with anything A) religious and B) 2000+ years old, there's a lot of disagreement on like every aspect of the above. But, I think I've got the gist of it correct from a generic, if Catholicism influenced, perspective. It's been a long time since I had to sit through a theology lecture.

With all that being said though, I imagine that the reason the OT has stuck around in Christianity is that it's characterization of God as vindictive and capricious and obsessed with toeing the line is a very useful tool for keeping the plebs compliant. They get to have their cake and eat it too, as it were. "God loves you unconditionally sweetie, remember that, but also if you have sex before marriage you are DAMNED to HELL for ETERNITY!"