Good luck. The Freedom Programme really will help you understand it and answer all these questions you have.
he threatened to ask our five year old if he wanted to see less of his Daddy
I'm sorry, but he doesn't love his children if he's doing this. They're just tools for him to use to get what he wants.
Can someone be emotionally abusive without meaning to be?
He denies he's abusive, refuses to change, refuses to acknowledge his behaviour is unreasonable, etc etc, not because he is being inadvertently abusive, but because he is being deliberately abusive and this is exactly how he wants things. He has no interest in changing, that's why he's never responded to your attempts to reason with him.
If he wasn't doing it on purpose you would have been able to talk him round. It's the fact he refuses to change (and all the rest) that tells you he's doing it on purpose. He has it justified to himself and he feels entitled to behave like this. Freedom Programme will help you untangle all this though.
As for leaving, I'm really glad you've realised you need to do so without warning him. Be as honest with the children as you can be. It's important they hear from you that "the way daddy is behaving is not ok and that's why we won't be living together anymore, but I love you all very much and I'm not going anywhere".
Please don't be tempted to give them excuses about how "daddy doesn't realise, daddy doesn't mean it, mummy makes daddy angry" etc. It will be much more upsetting and confusing if you do this, especially as they presumably know they're not allowed to behave the way he does. It would also leave the door open for him to manipulate them if you approached it that way.
They need a child appropriate version of the absolute truth of what's going on and the reason for it, that acknowleges his behaviour is wrong and unacceptable without minimising or excusing it. They most likely will have picked up on far more than you realise, and it will leave them confused if you make out like it's normal or somehow acceptable. Definitely don't say anything about you making him mad or sad or whatever, because that plants the idea this is your fault instead of his. Make sure it is all about his behaviour, not yours.
I don't doubt there will be some level of upset, but that will be about the change. You can give them certainty and security and honesty, and that will help. When they're older they will understand and be glad you were able to leave for them and that you protected them by telling them the truth.
I'm basing this on the info in Freedom, discussions with other survivors, etc, as well as my experience of being the child told "daddy doesn't realise" who grew up to think it was normal to be hurt and frightened by the people who said they loved you.
By the way, what he's doing is coercive control, so it's already something he could be arrested for.
You can absolutely turn this around. You really can.