I believe you have to do this one-by-one... the process is pretty quick though (unless you have 1000 of notebooks!). I had a notebook with 2000 notes and lots of notes in it.... the export was very quick. If you are leaving EN, you could consider consolidating your notebooks to make things quicker (but that may not work for you from an organisational point of view).
What are you going to move to?! I have tried OneNote, Notion, Bear, SimpleNote, AppleNotes, NoteJoy (among others) - and I came back each time....! That might be familiarity and history - but I struggle to keep with any of there others for more than a few weeks (OneNote was the longest)!