Go the proper route to get maintenance set in stone.
Block him on everything except a burner phone and dont reply to anything unless it's relevant to the kids and childcare.
He can message you all the hot air he likes about the new dame, just don't respond.
Ideally, have a family member do the pick ups and drop offs of the kids whenever possible. If you have to go then never go into his house or let him into yours. And just drop them off and go. Most of the time there won't even be anything you need to say.
Don't make the kids go to his if they don't want to. Let him take you to court for more custody if he wants. But in the mean time, fuck pandering to his demands regarding extra contact.
Turn your phone off and only check it once a day. If there's a message or reply you don't like, do not reply straight away. Put it down and go do something else. Give yourself time to decide if it's even worth replying to.
When you must speak to him, be polite enough but bored. And be boring. Don't tell him anything about your life.
He's feeding on your reactions. He's looking for your weaknesses. The less interaction he gets from you, the better.
Here's an example convo (in person at pick ups, where it can't be avoided via just not responding)
Him: 'how are you?'
You: 'oh im fine, thanks'
Him 'so my life is great now, Annie is just the best thing to ever happen to me'.
You: 'That's nice. So here these two. I'll see you on sunday. Byeeee!'.
Don't let things he says phase you. Just act like an acquaintance has told you a bit of boring news about a mutual acquaintance.
If need be, when he digs for information, give short, boring answers (I'm fine thanks) You can throw his own question back at him if you want to deflect from giving info about yourself away too.