Well, they have a small child now and very much aging great-grandparents who might not be in next year's Christmas pictures. And engaging with Prince Charles and Camilla sounds like fun. I think he'd be hilarious after a couple of tipples. It doesn't sound at all like this is "grey, cold, formal, rules, traditions". If anything, it has the warmth and personality of The Queen and Prince Philip. They both come off as having a rather modest style having come of age in WWII and both serving in the military, seeing their nation in great hardship and danger, respectful of their duties to the public.
I thought everyone got together with family for Christmas, as it is a religious and family time, then went on their sunny warm holidays after the New Year.
It seems to me, IMHO, that Megan is still early enough in her relationships with the Royal Family that she still needs to be there for these large private gatherings to respect and participate with these people. And bring Doria with her for support!
And, I think they are nuts having Archie miss out on this holiday at Sandringham.