How can I import all the hundreds of notes which I have in Notes.app into a database in Devonthink?
Maybe the first question is what export methods and formats does the Notes app support? (Sorry, but I don’t know!)
Look under File → Import → Folders & Attachments from Notes.
This will create a new Notes group at the top level of the database.
And individual notes are created as Formatted Notes when they are imported. These show up as html files, but can be edited from the editor pane.
Be aware that running the command again will import all the notes again. So you will end up with duplicates.
I imported my notes a few days back. It took about 15-20 minutes for about 2700+ notes (with not that many images).