Years ago I worked as an instructor for the company that is today American Flyers.
Most students usually arrived with no prior IFR experience, the instrument written completed, and at least the minimum flight experience required. 10 to 14 days later they left with an instrument rating. Training was 8 hours per day; 4 hours in the simulator, then lunch, followed by 4 hours in the airplane. The program was very professional, very structured and incredibly effective. I taught there for about 2 years and never had a student fail a check ride, in fact, check ride busts were extremely rare. DPEs were scheduled soon after your arrival and were never an issue.
Students were very well prepared before the check ride, and in fact had already flown a much harder practice check ride prior to the real one. Students are coached and orally tested constantly throughout each day. It was intensive and very manageable, but your attention must be 100% on learning for 16 hours per day, every day, no time for outside distractions! You will have homework most evenings.
These schools are very good at what they do. They are indeed expensive, but often cheaper than other methods because no time is wasted and no knowledge lost between sessions. It’s not for everyone, but if you are serious about getting an instrument rating and the confidence to use it immediately, this is the way to get it done.