Apple - Create local backup of notes in Notes.app on macOS
You may have copied the wrong directory, depending on your operating system.
Newer operating systems (I believe El Capitan and above, but certainly Sierra and above) use this folder:
~/Library/Group Containers/group.com.apple.notes/
Without iCloud there should be no issue with disappearing notes, which you would have to work around, if you were signed into iCloud Notes.
Apple's official recommendation to backup notes is to save them in a text/document file or export as PDF. From the Apple support article, Archive or make copies of the information you store in iCloud - Notes and Voice Memos
Notes and Voice Memos
Make a copy of a note on your Mac or at iCloud.com:
Open the Notes app and select the note that you want.
Copy and paste the text into a document on your computer, such as a Pages or TextEdit document.
Save the document to your computer.
You can also export a note. Just select the note, click File > Export as PDF, and choose a location.
The approach is a bit clumsy and doesn't let you backup all the various notes in one go.