To not want to visit in-laws so often

Three times a year in the main holidays is fine and fair to all.

It's so important for children to develop and maintain friendships out of school as they get older. This needs to override staying with family in half-terms now.

How old are the kids now? Are they going to keep being expected to visit when they are 16+? Is there much for them to do there? Three generations trying to spend whole days together is hard. The kids are bored, the grandparents can't keep up or don't like to compromise.

Things to consider depending on the age of the kids:
Shorter visit - 3 nights max so there is still time to do other stuff in the holiday. Get a few days or nights out planned with your kids or your friends so the trip dates get bookended and the trip needs to be kept brief.
Break the pattern with a half term trip to see someone else who lives elsewhere, a trip to Disney, a holiday with friends who have similar aged kids.
Any child taking exams soon they need to stay at home to revise for?
Let one of the kids stay home at a friend's house as 'they didn't want to come, they had stuff planned with their friends.'
Hope someone gets sick so the trip has to be cancelled last minute.
Suggest to DH you do 3 x 7 day trips instead of 6 x 4 day trips so you can have the half terms to yourselves.
Tell your DH he can go alone next half term as you want to just chill at home. Tell him you are becoming more of a home bird as you get older!

When you are there drop hints to the in laws that DS1 is starting to want to hang out with his friends in the holidays so we need to find some good activities to compensate. And that DS2 is missing a birthday party to be there. Ask if they find the noise and squabbling tedious - you may actually find they would prefer less frequent or shorter visits.

That’s a heck of a journey, having just done 5 hours each way to see my family this week. It’s practically the entire day with pee stops. I echo pp, your Dh can go and take a dc or 2, but only if they want. As a teacher, I told my Dh I didn’t want visits at half term, a week is barely enough time when you have to fit in everything like the MOT, shopping, dentist etc for yourself and the kids.

That sounds like a lot and really limits what you can do with your remaining school holidays. My in laws are a similar distance and we go twice a year, that’s plenty.

Every single school holiday does sound excessive. Visiting at Xmas, Easter and the summer seems more than fair so, no, you are not being unreasonable. I do however doubt that you really see more of the ILs than your own family if you actually broke it down. It probably just feels that way.

My family are a similar distance away and we stopped visiting in the half terms for the same reason. We go 3 times a year now. If there's another reason to go up I'll tend to go alone over a weekend.

Could DH not go by himself for some of the visits? Maybe taking one, or both, of the DC if they wanted to go on that occasion?
Or maybe all go, but just for one or two nights?