What makes an appointment reminder work
A reminder is not there to be polite. It is there to get an answer early enough that an empty slot can still be filled.
Timing does most of the work. A reminder the day before is the one that changes behaviour, because it arrives while the customer can still act on it and while you can still do something with the slot if they cannot make it. A reminder an hour before is a courtesy, not a defence against a no-show.
Channel matters because it decides whether the message is seen at all. A message on the channel the customer used to book gets read. An email to someone who booked over WhatsApp usually does not. Where the booking came in is the best guess you have about where the reminder should go.
The content should be short and specific: what, when, where, and one easy way to change it. The mistake is writing a reminder that only allows confirmation. If cancelling takes a phone call during working hours, the customer who cannot come will simply not come, and you find out when the slot is already gone.
Making cancellation easy sounds like it should cost you bookings. It does the opposite. An early cancellation is a slot you can offer to somebody else. A silent no-show is an hour nobody paid for.
Tell me what you are losing.
Thirty minutes on a call. I will tell you what we would build, and honestly whether it is worth it.