Handling a popular service that creates scheduling chaos because every job takes a different amount of time

Our most requested service has become a scheduling nightmare. One customer takes forty minutes, another needs two hours because every job has different complications. Fixed booking slots create delays and unhappy people in the waiting area. I'm considering a short intake questionnaire and two appointment categories, with buffer time reserved daily. Has anyone made variable-length services predictable enough to schedule without turning every booking into a phone interview?