DH has been in therapy, and I was starting to see real progress.
This seems like a parent talking about their child. A partner might say "he says it's really helping him" or " things have been going a little better around here" or " he seems more stable and able to handle stress".
OP, I think this is your problem. Watching for signs of progress means you're spending just as much time watching for signs of failure/regression. That's a really hard way to live your life.
Seeing and talking about your husband like this seems like you don't have very much respect or trust in him.
Toxic family dynamics are really complicated. You can only do what you can do with what you know right now. You can make a plan for yourself if something were to change but then you need to learn to live comfortably in the present, not the future potential.
If whatever happened with your inlaws is so bad that you cannot risk being part of that dynamic ever again, that's how you feel right now. Maybe in five years of no contact, you wouldn't care to show up at their family members funeral, who knows.
I can understand how much might be riding on maintaining this no contact, but again, so long as it's no contact, it's no contact and there's no way to get a guarantee of how long that will last or what it might take for it to change.