I'm not sure that my solution will be suitable for you. But here it goes in v10...
I highlight the notes that contain the PDFs I want to combine. I right click and select Export (not Export as PDF). I then select export as Single web page. The export proceeds and I open the directory. Inside there is folder which contains the PDF files. I combine them in Acrobat.
However, the current limit of 100 notes for multiple actions means that you would have to manually increase the limit for your account or do the work in batches of 100. You could the reconfigure the config.json file to, say, 900 and then you would be able to manipulate the 450 PDFs.