I have done a few meals for this sort of size group.
The roast dinner at the table is doable.
I like the lasange idea, as I try to prepare earlier.
I have made for starters in the past,
I seem to have a bit of luck with soups so make,
carrot and corriander soup, or leak and carrot, or leak and potato
Then either do a roast,(winter) or two big trays of chicken and ham pie, ( made earlier and frozen)
served with roast pots or mash, with fresh veg.
summer is all melon and parma ham, with fresh prawns/advocado , ( grandparents love the prawn cocktails)
main course has the baby new potatos and veg, and depending on starter have chicken thighs wrapped in bacon and fresh basil leaves with whole garlic cloves and good olive oil, a bit of italian bread goes well with this dish.
Puds are kept simple, I do make jelly and buy ice cream for the kids ( and often for grandparents who still seem to love this combo) but splash out on a exotic fresh fruit salad, or make a large choc ripple cheesecake.
I make a large pot of coffee and sit down sweating at the end of it all.
I have also had the joy of 17 for christmas dinner, which was hard work.
Not again. good luck you are going to need it.