What I want to achieve is boundaries not bitterness. So ex cannot upset me one way or another. I think I have that mostly. Occasionally I have an internal rant about the fairness of me being sole breadwinner and him absolving himself of parental responsibility but mostly I just focus on my life being good.
I think it's very easy to fall into the trap of thinking you must make things right. So you must adapt if he has work, you must worry about his relatives relationship with the children, etc
Boundaries are about recognising it's his life, his relationships and only if you benefit, should you do anything. He won't be looking out for you. It's quite liberating only bing responsible for yourself and DC.
With boundaries, the bitterness recedes. Bitterness isn't a good emotion so that's a good thing.