Notejoy makes it easy to import all of your existing notes from Apple Notes, preserving each note's formatting as well as importing attached images and documents. In addition, Notejoy lets you specify which folders to import and re-creates them as notebooks within Notejoy.
To begin the import, open Notejoy for Mac. If you haven't installed it yet, you can download it here. Then click Import Notes from the user menu in the bottom left corner of Notejoy.
From the Import Notes dialog, select Apple Notes.
Then click on Connect Apple Notes. Grant Notejoy access to Apple Notes in the resulting prompt. You'll then see a list of your Apple Notes folders. Select the ones you wish to import. Click Import to begin the process.
You'll get detailed progress on the import showing how many notes have been imported and the progress indicator will give you a sense of how many folders are left to import.
Your import is subject to your current plan's upload size limits, so you may not be able to import all of your notes and their attached images & documents unless you upgrade to a plan that provides adequate storage.
Troubleshooting
If you are not seeing all of your attached documents being imported, you may need to temporarily grant the Notejoy app full disk access. To do, click Apple Menu - System Preferences... - Security & Privacy - Privacy. Select Full Disk Access from the left column. Then press the Lock icon to make changes. Then check the checkbox next to Notejoy. Restart the Notejoy app and restart the import process and this time you should see all of your attachments uploaded. You can remove Full Disk Access once the import is complete.
Ensure you have Notejoy for Mac v1.11.0 or higher. Click the Notejoy -> About Notejoy menu to check your version. If you need to update, just restart the Mac app and wait to be prompted. Or you can download the latest version of Notejoy for Mac here.