Here is a different method you can use by taking advantage of being able to call the Graph API in a Tab application.The getSchedule Graph action (still currently in beta) allows you to query up to 62 days of Freebusy information on up to 100 calendars in a single call this makes it a good option for this type of application. So as long as users can view each others calendars or have detailed freebusy permissions the action should return the level of detail required for a Group calendar. The other thing you can do with the Graph API is get the users photo and build a nice legend for the Group calendar also. To make this visually appealing you need a good calendar display library which there has been a few over the years but a recent one that is really nice is FullCalendar https://fullcalendar.io/ . It ticks all the boxes it looks great, its easy to use and its free to use. To make the calendar appointments from graph appear in the calendar is as easy as building and array from the return JSON from the graph and throwing in a little random color code to break this up and then stitching in the user photos as they are returned asynchronously from the server to build the legend.
Here are some screenshots of the Tab in action using Graph data
I've put together a separate GitHub repository for all the code that is required for this app https://github.com/gscales/TeamsGroupCalendar and put some detailed install instruction in the Readme in Github.
I'm currently looking for work either contract or fulltime so if you need a creative developer