Event rejected by iCloud as 'quota has been exceeded'

Calendar and Reminders have started rejecting new entries to iCloud as 'quota has been exceeded'.

I have reviewed the help document –

iCloud: Limits for Contacts, Calendars, Reminders, and Bookmarks - Apple Support

which notes that the 'Total number of calendars, events, and reminders: 25,000'.

I have also been in touch with Apple support, and they have escalated the issue to the iCloud team to see if there is a fix, or workaround, and am awaiting a call back. However, from the initial contact I don't imagine they will provide a viable solution, not least as they regard iCloud a consumer product that one shouldn't expect to push too hard.

It is a challenge to thin out calendar events as they provide valuable history, and the only efficient way to thin them out is to archive a particular calendar (I have about 15) and then delete it. However, this will also delete any current and future events.

At present I am investigating subscribing to Google Calendar from Apple Calendar, and migrating some of the calendars with more entries to there. I am also unsubscribing from some public calendars to see if that reduces the number of objects being stored by iCloud.

Other suggestions would be welcomed.