Hi everybody,
since May last year we try to solve problem how to edit private records for full permission users at shared Calendar. Now I discovered a way (with Mr. Wild from grommunio helpdesk). Maybe it helps someone who did not now that, like me.
Change delegate access to private items
If you have assigned permissions to a delegate so that he or she can access your Outlook folders, you can hide personal information in appointments, meetings, tasks, and contacts. Open each personal item, and in the Tags group, click Private.
To grant access to your private items, do the following:
Select the File tab, then select Account Settings > Delegate Access.
Select the name of the delegate for whom you want to change access to your private appointments, then select Permissions.
Select the Delegate can see my private items check box.
Full description is here: https://support.microsoft.com/en-us/office/allow-someone-else-to-manage-your-mail-and-calendar-41c40c04-3bd1-4d22-963a-28eafec25926#__toc305134744