Thanks for the request. Currently there is a way to set this property through powershell for groups, but it only works for users in OWA. We have this in our plans for Outlook desktop as well and will be working on it.
We're a big fire and rescue station with 100 rescuers. We estimate Office 365 groups, but almost nobody needs write access. We urgently need to be able to set calendars to "read only", otherwise it will be chaos after a few days!