Oh thank you, and @Crikeyalmighty. I have actually been meaning to start my own thread on this so I shall soon. But a quick update: I am still living with him, out of financial necessity but, if I'm honest, a paralysis of proactive decision making based on a reluctance to make the wrong move. To be fair, I moved to 'his' country early last year in a misguided attempt to give it one last shot, so I'm still working out normal life admin let alone how to separate when my visa is predicated on my marriage status. But we have absolutely separated (rooms / beds / mindset) and he knows that my goal is to live separately and co-parent and he's coming to terms with that.
It got so bad last winter that I needed to see a therapist. I felt I was going insane. All those little paper cuts that we've all experienced that just sounded silly to complain about were threatening to consume me. I explored the possibility that he has narcissistic tendencies (that I appear to have brought out of him!), which has helped in some ways, because it isn't me... there is no solution, only creating distance so he can't hurt me anymore. It's a bit miserable making myself numb, but is better than trying to 'fix' this.
I'm doing everything I can to keep it amicable, which is mentally exhausting, but the arguing has lessened in frequency which helps things. But weirdly, also doesn't, because he then me feel like what we have is as good as it should be, until I remind myself that a) I don't want a life where my happiness is based on the absence of arguing and b) the arguing never really stops anyway.
So, while it has calmed down somewhat, and we can even have (a version of) fun and we rumble along peacefully enough at times, it's like I have a splinter in my foot, sometimes throbbing, sometimes ignored, but always there.