Is your husband feeling guilt and/or pressure? Did he move down South for you?
Do your in-laws approve of you?
How does everyone fit in with your parents?
Can you go see your parents sometimes when he goes up north? Can you stay home sometimes when he goes up north? With (preferably) or without the kids?
You need to work out what commitments you can tolerate as part of wider family life with husband.
You seem maxed out on commitment. I would never have got together with in-laws as often as once every 6 weeks.
(Around) Christmas, Easter, Summer BBQ and some birthdays, and the odd weekend, was my limit. Plus a few annual group family holidays.
But maybe involve some low and high tech.
Suggest your husband calls them once a week, just by telephone or online f2f video?
I suggested my husband met up with MIL midway once a month for lunch (about 45min travel time for both, she doesn't entertain)
He never really took to that one, once all the organisation and activity was his. It was to prevent his mother just turning up when she felt like it, without prior warning.
For the first 2 decades of marriage, I made his mother welcome, and I made her his responsibility. But her just turning up was a "no-go".
So set your boundaries. And work out a plan for the next 12 months.
Seriously, if you know when you're next getting together - It's something for everyone to look forward to.