An excellent (and fascinating!) point. I’m not an expert on Jewish theology either, but what you say makes sense. It doesn’t address the question of why God is sometimes harsh to his own people, and I think that question would almost certainly be addressed by pointing to the Book of Job (ie, sticking it out and suffering without complaint will eventually result in divine providence).
My understanding is that the Old Testament was written at a time when life was really tough; thus and angry and vengeful God. The New Testament – written in Greek, wasn’t it? – was written at a time when life was somewhat better, and there was a well-functioning society; thus, a kinder, gentler God. The Old and New Testaments were reflections of their times.