OP,
You are receiving so much good advice here.
Very wise to detach.
I have known people allow this to blight their lives.
As this is not new and they have form, in your place, as advised, focus on your husband.
This will likely be very triggering for him and early intervention is key.
Tell him how much he would benefit from speaking to someone and that it will benefit not just him, but your whole family.
Thankfully your family are a support so you are blessed, many have zero grandparents through disinterest or death.
I don't think going NC serves any great purpose BUT mirroring can be helpful.
Reply to a text but no more.
Cease to issue invitations or meet ups.
Leave it 100% in their court.
Do not mention them to your children.
What they have never known will not wound them.
In your place I would be very supportive of your husband, but also let him know that lots of grandparents are not hugely interested or involved in their grandchildren.
He is not alone.
It is not just him.
Uninvolved parents were highly unlikely to be hugely involved grandparents.
The people least important in all this are the grandparents.
Focus your energy and socialising on others and your children will not notice their absence.