OP it sounds like you have very rigid expectations regarding what makes a Thanksgiving celebration count.
-Be on the proper day
-Have specific dishes
-Include most extended family
The problem is that you can't control all of those factors. You can have your own meal at your house with the dishes and leftovers that you desire, but you can't make everyone agree to join you. You can bring the dishes you want to your in-laws and make that count. (I personally hate that option, transporting large dishes is a pain in the rear).
What does your husband want?
You know what? I think I do. I've been thinking about this. For the past 10 years, since we moved to DH's hometown, I have been celebrating every family holiday according to DH's traditions. I have come to realize that my family's traditions are disappearing, and I am not having the chance to share them with my daughters. My grandparents and parents are all deceased, so they aren't even around to tell my girls what our family holidays were like.
I understand MIL has the desire that our kids have memories of holidays at her house. But you know what? They will. She hosts Xmas Eve every year as well as Easter.
When I explained this to DH last night, he understood where I was coming from and thought my feelings were valid. We will see MIL tonight, so I would like to discuss it again.
Not surprisingly, I agree with you. I have been in EXACTLY your same spot. Including having a relative who said, "Someday I'll have to give this up, so I want to hold onto it for now." In my case, the end was definitely in sight--that relative's health was bad, and the very next year, she was out of the rotation. And, in my case the other holidays were not being hogged by one person. (And I don't see anything wrong with being a little rigid about Thanksgiving.)
I think your DH should open the conversation, and say, "Mom, we've decided--we're going to make dinner on Thanksgiving Day. We hope you will join us. I know you like it that your grandchildren have great memories of holidays at your home. But they are our children, and we want them to have memories of holidays in OUR home. We hope you and Dad will join us."
Then between now and then, feel free to make whatever casual mention is normal: "I'm going to practice chicken gravy this weekend," or "I saw a great brussels sprouts recipe Imight try out on thanksgiving."
Act as if it is a done deal. Because it is.
If you think MIL and FIL won't attend, then find other "family" and invite them. That will be the first step to claiming it. And when MIL and FIL invite you, say, "No, we have plans for that other weekend in December, we won't be able to come."
Don't talk about your boundaries--live them.
And maybe MIL and FIL will never give up the idea of hosting "the family Thanksgiving." OK. But your kids will have the opportunity to see how you host a holiday.
(though if I were DH, I'd be really tempted to say, in the follow-up conversations that are sure to come, "Mom, you're hogging all the holidays! You already get Easter and Christmas! Don't be piggy.")