We don’t eat out that often, but when we do, I look on it as an opportunity to appreciate good food and good cooking, not to seek to adapt a chef’s carefully planned combination of ingredients. As a T1 I’m fortunate not to need to restrict carbs, though I would restrict obviously carb-heavy stuff. So, if they’re part of the meal, there’s usually some potato or rice left on my plate, and if we get to the pudding stage (rarely, as we much prefer to have interesting starters) I just have a spoonful or two of whatever my OH is having. I don’t inject in advance of the food appearing because I have a better idea of portion size when it’s in front of me; I roughly calculate the carbs, inject accordingly, and correct later if necessary.
Chain establishments are certainly more predictable as regards carb content and portion size, but then, so is the food.