I built https://www.inngest.com specifically because of healthcare flows. You should check it out, with the obvious disclaimer that I'm biased. Here's what you need:

1. Functions which allow you to declaratively sleep until a specific time, automatically rescheduling jobs (https://www.inngest.com/docs/reference/functions/step-sleep-...).

2. Declarative cancellation, which allows you to cancel jobs if the user reschedules their appointment automatically (https://www.inngest.com/docs/guides/cancel-running-functions).

3. General reliability and API access.

Inngest does that for you, but again — disclaimer, I made it and am biased.