Please vet my startup idea - Event-driven automation for emails, push notifications and SMS

In my past 3 jobs, I've been faced with the challenge that can more or less be boiled down to the phrase "When A happens, wait B time and then do C unless D happens/has happened". A, B, C and D can here obviously be different things depending on the data-model we had in the app I was working on.

But. We never found any real good solutions. The solutions we came up with always ended up being something home-cooked that was expensive to implement, and expensive to maintain.
The existing solutions we found to work somewhat what we wanted to always had one or more of the below flaws:

1. Only concerned about a user (not it’s relational data)
2. Only works if the consuming applications data-model is replicated and kept in sync at all times
3. Very complex
4. Very expensive

I've been thinking really hard about this lately, and I think I've come up with a good solution to this problem. Something that is so flexible, that it works in all the scenarios I've encountered in the past.

I am therefore very curious as to how others are solving this problem. Imagine a product that can solve the below use-cases with little to no code, and little to no configuration:

a. Whenever a user is created send them a welcome email and notify us on Slack

b. Whenever a user schedules an appointment, send them a reminder via SMS and email 24 hours before their appointment, unless it gets cancelled

c. Whenever a user has completed an appointment, wait 7 days, and then send them an email to remind them to review their appointment

I very much look forward to your feedback, and please do not hesitate with any questions, if you want something clarified 🙌