As long as the client has a phone number in your db, you should be able to send them a message using Twilio.
I feel like having a button in the details view of the event would be a good alternative if you don’t want to set up something via the API.
Should be able to send an email to them, and only send when certain conditions match.
This is a bit tricky. In the future, you might be able to have scheduled automations in Glide, but right now your best bet is to use the Glide API through Make/Zapier, grabbing all rows, and determine which rows are “24 hours before the appointment”, then send a reminder SMS to them.
If you’re thinking about it as an event after the booking is made (with an action tied to it), then you can just tie a Twilio action to the end of that action chain.
I would suggest using email notifications instead.
If you mean through the notification, then it’s not reliable on iOS as far as I know.