So has anyone done inter-EU flight changes on their way out of the EU? How much hassle is it? For example is there a customs bags check, any legal requirements, or other issues? How time consuming?

Namely I want to do a London -> France -> US routing. But if going into France for less than three hours will be a huge hassle then I just won't waste my time. I wouldn't for example want a full bag search and have to pay duty to bring stuff into France just for a routing.