How would I go about editing a Pre-existing mob?

So for this instance, I am trying to change the behaviors of a Wither in order to make it stronger, more like the Bedrock Edition Wither. I understand that this kind of thing takes a lot of coding rather than procedures, but I'm willing to do that as long as you show me where to place the lines of code. (I.E. Do I create a new procedure and put the code snippet somewhere, or do I create a custom code Element?) If you need to know what it is exactly I want to do with the behaviors so you can help me better, basically, I need these key features:

1. I need for the Wither to actually explode once you've summoned him and his health charges. 2. If I'm correct, the Wither is kind of a joke in Java, he just spins around a bunch and fires off skulls everywhere, I would like to make him spin less and calculate where the skulls go. 3. Additionally, I would like to make him occasionally heal a bit of health. 4. Furthermore, I want it to actually spawn Wither Skeletons when he reaches half health. 5. Finally, I want it to explode once again when it dies. Most of these features are included in the Bedrock Edition for the Wither, except for the healing. I don't think it does that.

If you can help, thanks!