Appointment types with realistic lengths are the most effective single change. A review, a first consultation and an injection take very different amounts of time; without that distinction the morning backs up from nine o'clock.
Reviews after a treatment are the first thing to disappear. Six weeks after an injection or an operation the appointment needs to exist in the system rather than in the patient's memory.
Where waits are long, a waiting list is money. When somebody cancels in the morning the slot goes straight to the next people in line and gets filled instead of lost — on a six-week wait that isn't a detail.