Resetting end to end encrypted data

I've an old personal iMac (2011) and two work MacBooks, the newer of which is an Air (2020). In the process of setting up the Air (specifically when I selected Keychain in iCloud I think) my iMac said not all services would be available until I signed into iCloud again. I did, then provided the iMac Password. Then it asked for the old Macbook (2016) password. When I entered this it told me it was wrong (it wasn't). This went around in circles until the only option was to reset end to end encrypted data. I tried this but have had a spinning cog for the last 3 hours.

I've logged out of iCloud on both MacBooks.

It maybe connected, but the new MacBook Air was undiscoverable to either the iMac or the old MacBook (these were both visible on the other devices).

I tried Apple Support but they got no further than me and then said they had to hang up as the iMac was too old to try and help me for more than 15 minutes.

How long should the reset take and is there anything else I should try?