Want to sync calendars between Microsoft 365 and Google? I've had this problem and here's the only solution I've come up with after a lot of searching. It only works with a Windows machine and Microsoft Outlook.
1. Buy OggSync.
2. Install in Outlook.
3. Sync your Outlook calendar with Microsoft 365.
4. Run OggSync. This tool, installed as an add-in for Outlook, will sync your Outlook calendar (which is now pulling from 365) and your Google Calendar.
It works perfectly but it doesn't happen automatically. You have to have Outlook open to produce a sync.