Try disabling all those add-ons and add the calendar using the built-in CaDAV: File/New/Calendar, On the Network, enter the gmail address... calendars are autodetected...enter the account password in the OAuth window to allow TB access (cookies must be accepted in TB Preferences for OAuth to complete).

Thank you for your help, I now have a Thunderbird Calendar and a Google Calendar, neither is sharing with each other. So I guess the solution is to transfer everything in the Thunderbird Calendar to the Google Calendar. I will add all future events to the Google Calendar. Then I will hide the Thunderbird calendar. However this then hides my Thunderbird task list. Is there a way to sync the Thunderbird task list with the Google Calendar task list?

Not sure what you mean by the 'Thunderbird' calendar. Is it a local or network calendar? The Google network calendar syncs events between the Google calendar as viewed on the gmail website, and the same calendar as viewed in TB. A local calendar doesn't sync with any network calendar.

It's my understanding that the built-in CalDAV calendar only syncs events, but to sync tasks, an add-on such as Provider for Google Calendar is required. If you need to sync both events and tasks, the Provider add-on should work, but it might introduce unwanted bugs or features that are absent in the built-in CalDAV. If you don't need to sync tasks, I would create a separate local calendar for tasks only.

Since I don't need a local calendar I will transition to google calendar task list, then my laptop, phone and google will all play nicely together and sync. Thank you again for your help, I very much appreciate it.