Simple menu for 12?

I would go seasonal as much as possible.
Also choose an app, sides and/or desserts that can be made ahead and (some or most) served at room temp.
With 12 people, there's a lot of cooking, so concentrate on one dish (main) or at most two (main & app or main & dessert).
Seasonal on the produce...ramps, fiddle-head ferns, spring onions, asparagus, rhubarb.
One or two standout bread or pasta or dough crust for the main (retro - Beef Wellington?)
If you're splurging, a veal roast stuffed with pistachio pesto, or substitute lamb shoulder or turkey breast for the veal.

news.nationalpost.com/appetizer/bonnie-stern-green-crunch-goes-well-with-grey-weather
If you're not splurging, get something on sale at your local supermarket or butcher and make that the focus of the meal.
A very easy dessert is Eton Mess (basically poached strawberries with cream), but you could add rhubarb or use all rhubarb.
https://www.finecooking.com/recipe/eton-mess

I'd say go for big plates of a few things, with some garnishes to make them fancy. And, your secret weapons: butter, olive oil, and lemon. 1. Steam and plunge in ice water as soon as tender - or - roast asparagus spears. Serve prettily arranged on a big platter with a little olive oil drizzled on top, sea salt, pepper, a little parmaesan. Either microplane/finely grate lemon peel, or put twists of lemon peel, or just lemon wedges on the platter. 2. Good crusty French bread and good butter. 3.Cut good cherry or grape tomatoes in half. Gently cook over low heat with garlic and some olive oil. When they are tender put them in a nice bowl or rimmed plate. Sprinkle mint and/or basil on top at the last minute. 4. One big roasted meat and veg dish. How about chicken thighs (marinate the night before with olive oil, herbs, whatever you like) roasted with lovely thin spring carrots (multicolor?) and sauteed mushrooms and sprigs of thyme. You can use the pan drippings to make a sauce with butter and flour to thicken, and I would use a combination of white wine and chicken broth for the liquid.
You could do a green salad (mostly greens with a good viniagrette, it doesn't need a lot of other ingredients), or you could do cheese and fruit (a soft blue cheese and grapes would be nice) for a secondary course.
Put out some chocolates and make nice coffee and offer maybe brandy or Cointreau after and you're golden.