I might be talking too soon but the Notes sync issues I’ve had seem to be sorted for now. I’ve been on the phone with Apple and I no longer get errors in iCloud but had sync issues.
I took the radical step of deleting all of my notes in iCloud. I went to the site in Safari and deleted everything. This even goes for the notes In the recently deleted folder.
Once I had zero notes I turned Note sync on my iPad back on. After about 10 mins of syncing I had zero Notes on my iPad. I then deleted all of the folders in the app (you can’t delete folders on the web site).
I then turned on sync on all of my devices including my iMac.
I then started to create folders and notes on my iPad.
After about 30 minutes this sync’d over to all of my devices successfully and (touch wood) it’s still in sync.
Oh and make sure you have background app refresh enabled for Notes across your iOS devices.