“I’m even questioning the emotional abuse as he is being so nice today. It seems like this a lot, I want to leave and I guess I start being distant and he starts being nice. Then I doubt myself wondering if I’m imagining how bad it is. It’s been like this for years.”
Have you read Why Does He Do This? by Lundy Bancroft. My understanding is that abusers do behave kindly and lovingly at least some of the time because more or less no-one will tolerate endless vile behaviour. But if your oldest child is saying they don’t want to live with their daddy, I’m guessing there is sufficient unpleasantness to make life uncomfortable much of the time.
It may seem harsh to leave him w/o warning but a woman in an abusive relationship is in greatest danger when leaving and in the immediate aftermath, so it is the safest thing to do. And abusers who have not been physically violent can turn violent if they realise they are losing control.
From what you say, the marriage is over and the priority must be to get yourself and your DC out safely. Get as much RL support as you can. Gather all the vital items such as bank cards, medical books, passports etc. and some clothes etc for the DCs. If you do decide to tell him, make sure you have at least one other adult there for support who can help if things get hairy.
Alternatively, perhaps he could be the one to leave? You haven’t described the abuse but perhaps it would be sufficient for a non-molestation order. If that were granted, he would have to leave the home, at least temporarily.