Shared calendar doesn’t sync between devices – strange errors and error messages

My wife and I currently have a shared Google calendar that we access via Apple Cal, but we remove Google.

I set up a shared calendar on my iMac, invited her, and she joined. The calendar appears on my MacBook, phone, etc.

We needed to migrate our existing shared Google Calendar entries – including recurring and all-day events – so I exported the calendar file from Google Calendar and imported it into iCloud. It seemed logical and the online guides recommend it.

What’s happened since then is…

— my imac had all the events, but my macbook only had some, and so did my iphone. There was no pattern to it, for example: it wasn’t just a matter of a lack of repeated events

— doesn’t see all the events I’ve imported

— A 503 error and a refresh failure message appear on macOS Calendar

— I logged into icloud.com and not only was I greeted with login issues and a maintenance error, but there was literally nothing I had imported on the calendar screen

Has anyone had a similar problem and can recommend a solution? It’s a strange combination of Google Cal, perhaps not exporting every event, but then icloud not syncing between devices in the way you might expect