I’m in the same station as you, probably abit worse because I’m pregnant and married to him
We have a group chat with my step sons BM, her mum, myself and my partner (main carers) which we use but my partner rarely messages into it, the majority of contact is through me bc we found it causes less issues - there’s an occasional private message between myself and BM too depending on the subject
Buy another cheap phone and SIM card and give her that number.. that way you can just leave it in a drawer until you want to look at it and that way she won’t be bothering your phones at all!
We blocked WhatsApp and only leave text open for messages.
We are all living the same nightmare 😭😂 tbh unless it’s about the child my partner just completely ignores her and just doesn’t engage for his own peace of mind. Let her argue with herself!