I've sort of moved away from "fast food" and mostly hit the sort of new fangled "order then we cook it" places like 5 Guys although the price of those places generally makes me hesitant. I can buy 3x greasy burgers for $15 and eat some borderline toxic fries or go get a pound of much better quality beef and some low carb buns for like $6-7.
Probably taco places are our current fave, Taco Fresca is a chain but they opened a new, independent place (Taco Diner, if you are in the Naperville/ Bolingbrook, IL area, very tasty stuff...) about 5 minutes from our house that has very good salsa, tasty tacos and other goodies, pretty much the whole menu, it takes 5-10 minutes to get a take out order. My main concern is that the food is tasty enough that if they get bigger, they will outgrow their hole-in-the-wall and move somewhere less convenient. I usually go w/ about 45G of carbs for 3 tacos and only eat one tortilla and run a shade low afterwards.