Sadly, after a little more work with CPN4 in Catalina, I've found it to be crashing again. Considering AquaMinds NoteTaker. They say this about importing from CPN:
Not directly. However, NoteTaker 4 can import RTF, OPML, HTML, and (to some extent) NBML files.
You must first export or copy your CPN content into files in one of these formats. Then create a new NoteTaker 4 notebook, and use the File->Import… command to import the files. Each of the formats has advantages and disadvantages.
NBML may preserve the most formatting and metadata, but in our limited experience, CPN can only export a single page of NBML at a time.
On the other hand, CPN's OPML export may allow an entire notebook's data to be imported at once, but formatting and attachments will be lost.
You should experiment with transferring your data in all of the available formats and choose the one that provides the best results for your content.
In general, if you import several files at the same time, they will all be placed in a single section. When importing NBML, NoteTaker 4 will attempt to also import attachments. It may prompt you for the location of the folder containing the original attachments. For embedded attachments, this is a folder called “attachments” within the original notebook.
To view it, control-click on the notebook in the Finder, and select “Show Package Contents”. You can then copy the folder out of the notebook where it can be accessed by NoteTaker 4.