Oh well...... no meaningful responses here, and no progress on fixing the bug whereby Sketch handwritten notes hang when you try to save them.
I've switched all my notes from Evernote over to Notability. It took me a few hours but I can now write notes with my Apple Pen safe in the knowledge that they will be saved.
In order to export my Evernote notes - I just printed them as .pdf and dragged them into Notability.
So long Evernote. Much as I love the Evernote app, it has been allowed to go off the rails big time. Also the company has not invested in resourcing customer service to a level appropriate for a subscription business. People who pay annual premium subscription deserve attentive customer service, and the removal of showstopper bugs within a reasonable timeframe.