You cannot make the other parent reasonable. What you can change is how much of their message reaches you, and what state you are in the next morning — because that is what your children actually feel.
Unless a child's safety or health is involved, nothing has to be answered tonight. Write the reply if you need to get it out of your system — then leave it in drafts until morning. You will change it.
A few sentences. Facts, not motives. Not warm — just not cold. Close the topic instead of opening three more.
Notice what isn't there: no defence, no counter-accusation, no reference to the last six things.
Five grievances get five defences, and now there are ten open threads. Answer the one thing that needs a decision; raise anything else separately, later.
If a message is designed to provoke, the reaction is the point. Answer the practical question promptly and plainly, and withhold the rest. Done consistently, this changes the pattern more reliably than any argument.
The veil holds a difficult message until you choose to read it; the summary tells you what it is actually asking; a calm draft is waiting when you open the thread. The techniques above are what the app automates — but they work on their own, too.
None of this is for threats, intimidation, or fear for your safety or your children's. That is a matter for a lawyer and, where appropriate, the police — not a better reply.