MacOSX calendar bug

Hello

My calendar has become quite buggy : the most annoying aspect is that details stored in the 'notes' section of an appointment don't always 'stick'. I often create a copy of an historical appointment, and then paste it into the present - and then make modifications to the 'notes' section of the appointment. Quite often, the detail contained in the present appointment's 'note' section, revert to the historical appointments 'notes'. Another issue : sometimes, upon creating a new appointment, it is impossible to move or adjust the duration of the appointment by dragging/dropping or stretching/compressing. The only way with such 'stuck' appointments is to adjust their parameters from within the appointment itself.

I am using Version 11.0 of the Calendar app on a 2019 iMac. I used the calendar for both personal and work appointments, however the bulk of the appointments are work appointments of my 2 employees + myself. In total, I have 12 calendars - where 7 of these are shared. All of the calendars are iCloud calendars.

There are a large number of appointments - perhaps an average of 10 per day, going back 10 years - so that's potentially 36,000 appointments. I'm wondering if it's this that's causing the issue - and that the calendar is 'overloaded'. I've got a 250gb iCloud account - it's about 50% full. I've logged out of my Apple account, and back in again (several times) as well as restarts etc - nothing seems to help.

If the opinion is that it's overloaded, is there a way to purge certain events (perhaps events older than 5 years) to lighten the load?

Any help would be much appreciated.

Matthew