What are you going to do then OP? Ban him from occasions where wicked witch of the west ex-wifey will be present? So no -
School Meetings
Graduations
DCs weddings
Christenings
& so forth
Only you sound pretty self-assured that once his kids are grown you'll have no dealings with her. YOU may not, thats true - but he will. & its nothing to do with 'entitlement' - just the life of 2 people who share children together. It is what it is. Theyll share grandchildren too by the way
& if you have children then theyll be brother/sister to the children he has with his ex. I split with my DH many years ago, we are amicable now. His 18 year old DD via his current ltr, is upstairs in my home with the 3 I had with him, right now. Theyre siblings and want to spend time together, go out etc. & Im here.
Even if his current wanted to, she couldn't prevent that. So...I am around their children's lives too. They know me well enough and have all spent time here. Thats what family life is, better to get on with it.
Me & kids dad come across each other pretty regularly and the sky doesnt fall in. What are you going to do when you get to this stage?
You will make life difficult for yourself but if thats hilarious enough for you, crack on.'