Hi, I fully understand you not wanting to do a traditional Christmas Dinner with the full works. I think you need to sit down and have a proper chat with the family. If they want to have a full on Christmas dinner then tell them that they need to realistically plan it, shop for it and cook it from scratch.
Just tell them that you need a complete break from doing it this year and that unless they do want the buffet you suggested then they need to sort the arrangements as you won't be doing it.
They just don't realize how much work there is because you've obviously done it all for them before.
I think you are going through Christmas Dinner fatigue. I have a similar issue myself atm abit like yours but for slightly different (and self inflicted) reasons.
Apologies if we sound over the top but we are definitely foodies, my kids have been brought up growing alot of our own fruit and veg, collecting eggs from the hens, and trying and eating lots of different things from a very young age so they are very adventurous and eat most things.
And they've grown up with me cooking most things for Christmas and New Year from scratch, so that's homemade mincemeat & mince pies, Christmas pudding and cake, buffet food, stollen, homemade stilton and walnut biscuits, shortbread, lots of deserts, icecream, port & cranberry sauce, stuffing, marmalades and chutneys and pickled shallots and beetroot, pickled pears, the list could go on.... yep you name it I've probably done it. But I do enjoy cooking especially unusual things provided I'm not being forced to do it.
I obviously didn't start off immediately doing all that, it gradually crept up over the years whilst I was a SAHM and because family and friends enjoyed it, it became the expected norm. Same with actual Christmas Day although we've never been a turkey family - not keen - prefer Duck or if visitors Goose.
Well last 6 years we've had visitors who now expect the full Christmas thing completely homemade, and turn up their nose if I try any shortcuts - 'Oh when you said Steak and Kidney pie I thought it was one you'd made, oh you bought a ready cooked gammon.....' But of course I'm now working and am doing that even when they come and stay for a week or week & half over Christmas (I also normally work Christmas Day, Boxing Day and New Years), so I have alot less time. Its gotten to the stage that every year I was having to start preparing things and freezing meals for their stay in September - I kid you not! They are also traditional and picky eaters so no rice or pasta, lots of roast dinners, casseroles and pies all time consuming things to cook.
It had gotten to the stage that although I loved seeing them, and enjoyed some of the cooking, I was dreading every Christmas as I'd end up slaving in the kitchen and feel exhausted by the time the end of their stay. Even where other members in the family would help out with some food prep for the big day, I would spend most of the time trapped in the kitchen cooking, washing up etc etc.
Anyway this year we can't have visitors due to certain things that have happened in the house. Nothing drastic, but enough to mean we physically can't have anyone staying as most of our downstairs is not liveable.
I basically said to the family that I wanted to do an easy Christmas and New Year. I don't want alot of hassle.
I did initially say that I wanted to eat out Christmas Day, but the prices are stupidly rip off expensive, when I know for a fraction of the cost for a family of four eating a bulk standard turkey dinner down the pub, we will be able to eat a really knock out meal cooked at home. So its going to be things that the family wanted to have other years but couldn't because of our visitors. And the family are going to cook it completely this year.
I've also said I am absolutely not doing any of the preparatory cooking. So the pudding is one I did last year (always end up doing 2 at same time), but not making a cake and icing it, no mince pies, biscuits - etc etc. I've bought a Panetone and a stollen, and if they want anything else then its going to be a Tescos job. I will not make a single dessert or mince pie.
And I'm doing to thoroughly enjoy being completely lazy when I get home from work on Christmas Day and the only thing I'll be looking to lift is a glass of wine!
So OP I would seriously say to you, go on strike. Tell your family that you have Christmas Dinner Fatigue, and that they are welcome to do the traditional Christmas Dinner if they do all the cooking! Otherwise its your buffet (but I agree with other people that a buffet can also be quite labour intensive.
Good luck!