Hold on, a brioche bun can totally work! Toast the bun, put a little mayo on it, put the veggies on the bottom (at least the lettuce), and a regular-sized burger will hold up just fine.
Not saying it can’t go wrong, especially in a place that just wants the decor and the food to look good on Instagram even if it’s disappointing when you bite into it. But for burgers I’ve made, a brioche bun can be a nice option. :P