Thanks for this, and also to @Petra42 @Apex3 and @BustyLaRoux for the ongoing good advice about dealing with my oppositional DP.
I see one scenario that might possibly work. DP has recently made massive parenting improvements by taking accountability for how he has parented so far and really improving himself, and his childrens behaviour. I don't see his kids much but when I've done so recently, they have been a joy to behold.
DP recently woke up to the fact that he had been teaching high-conflict communication to his children and this was a big reason why both have turned out to be quite challenging.
He has told his children that one reason they had been in so much trouble and experienced social rejection was because he had showed and encouraged them to question authority and ignore hierachies. This resulted I think in the kids' belief that they were inherently bad starting to lift.
Another thing these kids have always lacked - their parents' relationship was toxic - is any exposure to healthy adult partnerships that model self-awareness, kind communcation and compromise. If they continue to lack this, they will become adults, like DP did, who have no idea what a good relationship is supposed to look like (his parents, like mine, were high-conflict).
So I plan to tell him as gently as I can - and believe me this will be my very last effort before I leave him for good - that learning, practicing and embedding better communication skills, accountability and awareness with me will be a massive gift he can give to his children. They will see something good and learn from it. DP's kids are the centre of his world and he will definitely work on himself for them, I know, because I have now seen this.
DP desperately wants me, him and our joint children to live as a family. I have refused this on the grounds that our communication is unhealthy and I have to shield my own child from growing up being influenced by how DP talks to me. I have also for the past year or so arranged the 50-50 rota with my ex so DP and I are with our kids and kid-free on the same days.
But what I think I can do now - and believe me this will be the very last effort before I give up - is tell DP he can have the 'family' he wants as soon as he is able to relate to me in a manner that shows our children a healthy adult relationship. He will want this for his kids, I think I can convince him to realise, because otherwise they will become adults who have no idea what a healthy relationship looks like, just as he did and just as I did.
So I am now planning to tell him this, on a day when he is calm and regulated, and see if he thinks it is achievable. If he does not, or argues about the strategy, then that will be my cue to leave, block all contact and change the locks.