I wonder if taking a seemingly cooperative attitude to 50/50 would work?
Absolutely. I'll train you how to do it all. Here's their timetables, diary, baby feeding schedule, medical needs, social things, homework, bedtime …
Making it absolutely clear exactly how much extra work he'll be taking on that he never did before when it's just him with the children cramping his style.
All the time pleasant, businesslike, cooperative, reasonable. Giving nothing to latch onto or accuse.
And document and keep everything.
Screenshot every text and social media message.
Record every phone conversation and message.
Keep every email, letter and other corres.
And put your phone discreetly on record when you speak with him.
Capture threats, attempts to gaslight, deny, accuse.
Tone of voice, choice of vocabulary can be very relevant.