Full Calendar Management for all users

I am building an app where I need to manage the availability and booking for all my users. (Time slots won’t work… it needs to be able to allow booking directly off a custom availability.)

Is anyone familiar with a Calendar platform I could connect to and achieve this? Was looking at SuperSaas or something similar… Looking at their API documentation… (Developer | User API) it looks like I would be able to create users, associate them, manage availbility and booking. But after talking with their support team, they said the API is really supposed to be Read Only.

Anyone have suggestions for a platform I can connect to using Custom Actions on such a large scale for managing hundred or 1000’s of calendars?

Or maybe some of the following: