There is a macOS app called Exporter. Search for "backup Notes" in App Store.

EDIT: I just tried running it, and nothing happened. Might be a beta macOS Monterrey problem.

EDIT 2: this just worked for me, needs an iPad, but copies Notes as markdown files that then also show up on my Mac: https://wimpostma.com/blog/how-to-export-and-backup-notes-fr...