Thank you for creating a private and functional Calendar. I use it for family and for my business. As you continue to develop it, have you considered a phone number built in so that the users can text reminders to customers? In my business model, I use Proton Calendar because it is private and secure. Most of my customers (patients) would benefit from text reminders/notifications rather than notifications through email. Consider enabling text features for the same options as the email/push notifications that are already built in.
For example, Jane Doe schedules a session/appointment Thursday, 8:00 a.m. I schedule that in PCalendar. She receives the invitation same day as scheduling through email (that may or may not sync with her chosen calendar). Then 2 day prior on Tuesday at 8:00 a.m., Jane receives a text reminding her of her follow up session/appointment.

I'm choosing to use PCalendar for it's security and privacy and don't want to subscribe to an additional outside of Proton service to complete text reminders. I currently manually send out text reminders for each session and hope PCalendar can create text reminders into it's system to streamline business efficiencies. I've added customer emails and phone numbers into the PAddress book already.
Thanks for your consideration. Cheryl dba Chay Mission