And that's really kind of you. I'm just trying to understand really. I do have some points of reference that may help me put myself in your shoes but as I don't have any obvious answers I don't know how much use that is!
But a lower opinion of him is a significant threat in itself, and what he's done would certainly bring that about.
Yes, this might be a possible approach. Can you "threaten" him with that in some way (but make sure you're safe first - don't ever forget to be careful!)? If you had even one person you could tell this to IRL that would make a huge difference in terms of your safety. I'm convinced he's playing a double role here, and fear of others seeing through him could have an effect (but is also why you need to make sure you're always safe above all else!)
I think there's also a sense, from him and from them, that I'm not easily intimidated and can look after myself ... more fearless than I really am
That's really relevant in terms of your family's opinion, for sure.
For him, he had no idea how terrified I was when he was trying to kick the bathroom door down. Obv most women would be, but I do wonder if he thinks - well, she's not like that. She knows I wouldn't do anything worse. She's seen worse. And in some of these incidents, my presentation has been more along the lines of "who the hell do you think you are?", until I switch into this dissociation instead. I probably don't look remotely afraid at the time. He thinks I am difficult to intimidate (not that he's trying! I mean, he sees me as someone who can stick up for myself, and has a lot of moral clarity and moral courage. Like, how could she - who grew up with a violent addict - be afraid of me?
I suspect he knows it more than you realise, because why does he only do it when there's nobody around? And you said he'd be annoyed if you showed that you were afraid of him - so that proves that he can tell himself it's ok as long as you don't show the effect it has on you. He can minimise it because you're so strong - but in reality he knows what he's doing.
He threatens to leave you if you tell anyone - but leaving would put him in a bad position, right? Being the one to cause the break up? So I think he's calling your bluff on that. I think you need to think about that, see if there's anything useful there. (I don't want to make any suggestions that could make things worse - it's just an angle to think about.)
Different question, if I may: you've been together 20 years. I don't know if you've said how old your children are, but if they're mid to late teens, I imagine they must know or suspect something? (Don't give out info that might identify you, obv - the exact answer doesn't really matter. It's just if they're all under 10 it's not the same as if they're 15-19 for example)