I think, OP, that you are getting a very hard time on here. Especially as you are the one who has ensured, in the past number of years, that DGran HAS a nice, and hot, freshly cooked, Christmas dinner served to her.
Meanwhile, your DPs, your 2 DSiblings and their partners and DGGCs, and various other DAunts and DCousins all get to take a step back. Either going out for nice meals cooked by a chef, or just visiting but not needing to think about DGran getting a Christmas dinner.
I think, given you have done the running around (including taking an hour driving in the middle of Christmas day, and no doubt it's not just a "Hi Gran, here's your dinner" and dash back out the door visit, so even longer away from the house and YOUR family, helping DP, playing with DCs, enjoying the holiday yourself....) that you have a right to say "I've done my turn, now it's someone elses".
Maybe put it out on a family whatsapp group that you are willing to still cook and plate up DGran's meal, but you realise you need a break as a family and won't be leaving the house this year so someone else needs to collect it when "going to visit DGran anyway". Or else that someone else needs to organise it entirely.
Just make it clear that, this year at least, YOUR boundary is that you will not be leaving the house on 25th. You will visit DGran at a time (or times) over the holiday season that suits both you and her, but that's a separate matter for you 2.
By saying it early, and letting them know you are happy for them to collect a plate for DGran - you're not expecting them to organise it ALL (although that would be nice), it shouldn't be a problem. There is still time for reservations to be made larger to include DGran at the restaurant. Or for people to build in time to make a stop at your house on their way to see her.
But just lay down the marker that - at least this year given all that you and your family have been going through - you will not be leaving the house so cannot deliver DGran's dinner to her. So who is going to do that or what alternative plan does anyone have for DGran?