I actually agree with your husband.
- his wife got the house, he walked away with nothing
- you had a bigger house deposit
- he supported the kids through uni anyway (was he able to do this for the kids you had together?
- you have built what you have together, over 40 years.
- and I think you should definitely ring fence the inheritance from your family for your own children.
At the end of the day, his kids will have a house to inherit from their mother, and a payment from your estate (and it is YOURS). Your kids inherit only from one. If their mother fails to protect them in a will, that's on her, not on you. Don’t feel guilty, and don’t do your own kids an injustice because of guilt.
I would go along with the amount you have discussed with him, and make sure that all the details are with the solicitor (including that you put a bigger deposit into the family home, that their mother got the house and it’s up to her to protect it for her kids, that their father supported them, that as it is, the step kids will inherit from 2 estates, your kids inherit just from one. And that it was discussed before he died. All the details.).
And as you say, if you need care, there may be little left, and your kids, who may have had to step in to help you, will get even less.