Brownies with vanilla buttercream and chopped up candy canes sprinkled over is the bomb. The Ghirardelli mix brownies are great for it.
I make Christmas pudding every year and adore it but it’s an acquired taste so wouldn’t recommend it.
A nice way to do trifle is individual trifles in plastic glasses (plastic champagne flutes are fancier). It avoids the problem where the trifle starts looking disgusting after a few scoops are taken out. My favorite trifle is: chocolate or vanilla cake; homemade vanilla pastry cream; mixed berries with strawberry preserves; sweetened whipped cream. By the time you later it all in, it’s not a big deal if you use box mix cake as the base. But don’t use boxed pudding — the pastry cream taste should carry the day here. For an adult crowd, you can do a rum syrup to pour over the cake. If you make in advance be sure to stabilize the whipped cream by adding a little dissolved gelatin (which is not kosher or vegetarian, so may not work for all guests).
This is my favorite easy buche de Noel—one of my kids found it when they needed to make a fancy dessert for a party and it was really delicious:
https://www.bhg.com/recipe/chocolate-cakes/mocha-bche-de-noel/