I don't see the problem with the Google CalDAV calendar in Thunderbird
68.3.0 on my Ubuntu 18.04.3 LTS Linux system.
I just created a recurring event that started today at 9 pm and had it
repeat daily until next Wednesday.
Clicked on the event and the "Edit Repeating Event" dialog window opened.
I selected "Edit all occurrences".
I edited the start time to 8 pm.
Clicked "Save and Close".
All occurrences updated without any problem.
Selected the event on Saturday.
Selected "Edit only this occurrence".
Edited the start time to 10 pm.
Clicked "Save and Close" and the event was saved.
I'm not a big user of recurring events. I have one that occurs on Monday
and Wednesday. I just drag and drop it into the next week on those days.
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OS: Ubuntu Linux 18.04LTS - Gnome Desktop
https://www.thunderbird.net/en-US/get-involved/