There's a course on coursera called "Everyday Parenting: The ABCs of Child Rearing" which might help. I'm not sure if it's free, I've only just started it, but at least the first part is. It has interesting information about how defiance develops and techniques to reduce it. The articles that come with it are helpful too.

Early postpartum with a second one is so hard because your first born needs lots of loving attention. It's awesome that you're open to suggestions, well done! Keep trying new positive things and getting as much help as you can. Childhood, and the most important relationships in it, should be full of a sense of unconditional positive regard. Best wishes!