NP here. Sometimes you have to accept that you will see DH's family the majority of the time unless your parents retire and move closer or you end up moving back. My family situation is different that while my family is maybe 5 hours away, my parents are divorced and my sisters aren't speaking to each half the time so getting everyone under the same roof here for even a birthday celebration for my child is a challenge. This doesn't even factor in the money aspect of getting here as in I would need to get a plane or train ticket for one, a train ticket for another, maybe a bus ticket for another. This also doesn't include the fact everyone works except one person while my in-laws are retired. Going there for the holidays is a non-starter because we would need to stay in a hotel. The only one with space is a hoarder. But despite all the issues I love my family and I do feel guilty that we don't see my family for the holidays ever.
So out of necessity I have adopted the attitude that the important thing is that you do spend time with your family and it doesn't have to be the holidays. I have tried, some years successfully, to have my mom visit on her birthday. Or maybe as someone mentioned you vacation with your parents around 4th of July and that becomes your tradition. If your parents are truly alone as in you have no other siblings, they have no other siblings and their parents have passed, and you are literally the only relative they have, then you may want to think about if they would end up moving to be closer to you once they retire. Likely that will be the only way to spend more time with your family and in that situation I think it is reasonable to expect that you would see them at least as much as you see your in-laws.
As for you seeing your DH's family after Christmas when it is your family's year for Christmas, I think there is an inherit unfairness. Personally, I think if your in-laws want to see you after you either just got back from staying for the holidays or your parents have come in town, they have to come to you. I also think if you are basically going to see them at some point around the Christmas holiday every year, it is fair that assuming your parents have the time off and can come see you, they get the same deal in the off years. Christmas means Dec 24-26th and if it isn't your year and you can come see us, you can come before or after and it may mean you overlap with other relatives coming to see us. A your parents may not be able to afford to do this every year, but should have the option. If this causes more drama then you stay at home and you host. I would have started that policy on the year you go to your parents because I think you have the moral high ground saying look, I just came back from XYZ, I would love to see you around the holidays but I'm not up for more travel, you are more than welcome to come here. Then the following year say look, we are having my parents come to spend post Christmas with us, just like you got to visit with us last year after we went to see them. I don't think the 9 day visit including Christmas is going to work because that isn't the same as seeing someone after or before the holiday plus that is a long visit. If you want to see your parents for 9 days I think you need to plan a vacation together.