So a man who sexually and emotionally abused you, has got in touch to abuse you and scare you by calling you a 'terf', and you are worried that he will call you a 'terf' aspublicly?
First of all, even if he does, so what?
He'd be pretty much demonstrating the fact he's abusive and that what motivates him is abusing women.
If your friends decide to value his opinions over you then they were never friends in the first place. They should be willing to listen to you say he was an abusive ex and I have no idea why he's got in touch to abuse me again.
He is harassing you.
And besides this, your husband knows the history and should be your champion here to anyone who decides to be difficult.
Imagine you had gone to a women's refuge because of his abuse. If you weren't a 'terf' you would be falling to recognise your own abuse and the risk some one like him would pose to a woman if he could just claim to be a woman and stroll into the refuge too.
Being a 'terf' isn't a bad thing. Indeed many women labelled as such precisely because they recognise or have been the victims of abuse and are trying to prevent it. This makes them a threat to men like this. Not because you will deny their existence or refuse to validate them. But because you see them as what they are - abusive men who will never ever be women. And they hate you for it. They hate the women who remind them of that.
When he looks in the mirror he knows he's not a woman. When he actively tracks you down after years of no contact purely to tell you he's trans, he's doing so precisely because he knows he's not a woman and he wants to assert power over you as a woman. That's not the actions of a woman. That's the actions of an abusive man.
You have nothing to feel guilty or shame for. Being called a 'terf' isn't something that you should live in fear of. Precisely because it's about him and power and control.
He's demonstrating the validity and importance of being a 'terf'. Be proud of that and take ownership of the label. Being kind should be about mutual respect. If he's not respectful of you, he's not kind. You don't owe him kindness and none of your friends should expect you to owe him to 'be kind' purely because he's decided to come out as trans. Remember that.
I always say, focus on behaviour and not identity. If behaviour would be unacceptable coming from any other person, then it's not ok. And that's why we shouldn't just blindly give males who don't respect women's privacy, dignity and right to define themselves, this status of being victims. Any person who doesn't recognise why women need single sex protections and sees males who are happy to impose themselves in this situations is not an ally to women... They don't value women. They see women as second class and just there to validate men. They see women as not worthy of having their own traumas acknowledged and recognised.
Ultimately I'd frame it like this for your own sanity:
Your ex is not trans. Your ex is an abusive male trying to abuse you once again whilst hiding behind a skirt to protect him from being called out for being an abusive man. How can you tell the difference between 'true trans' and an abusive male pretending to be trans? And that's why you can't be anything but a 'terf' after he's contacted you out of the blue to abuse you.