It sounds to me as if ex had this lovely (theoretical) image in his mind of his shiny new life. One where he was footloose and fancy-free, could do what he wanted, when he he wanted, without any responsibilities. He conveniently forgot, if indeed he ever knew, that parenting does definitely come with responsibility, requires time, money and importantly, putting someone else first other than yourself.
He also, I think, envisaged a world where you’d just agree to whatever he wanted and had not pictured any scenarios where you’d just say no, not happening.
He’s already realising that his dream of waltzing off unfettered and on his own terms into the sunset ain’t happening. His theoretical new life where his children would ‘arrive’ as passive packages requiring no input (no bedding / nappies / fishfingers for tea / homework / waning up etc) and be returned to suit his own selfish desires is already disintegrating, largely because I don’t believe he ever understood what being a proper parent means. He always stood back and let you do it all. And called you controlling when you dared to suggest he might actually get involved in family life!
Like many other posters here, I will be astonished if he ever delivers on his 50-50 pipe dream. He is too clueless, selfish and unprincipled.
I’m glad you are finding your anger. You can now channel this into securing what’s best for your children and you.