So having this same issue, I stubbed across this thread... playing around with this I was able to get this to work and boy does it work fast!
1. Arranged my 600 pdf documents I wanted to merge (each is about 20 pages) into a folder in Finder and sorted them by name in the order I wanted them.
2. Clicked on the first one only, and import into notability into its own note
3. scroll to the bottom of the note just created in step 2, then select file, import
4. selected the resining 599 pdfs from my window that was sorted and drag and drop the whole mess to the bottom of the last page of the first note (in step 2), then release the mouse button
5. In about 3 seconds literally, all of the notes are merged into one document.
Then they replicate through iCloud, as I have that turned on in notability. Boom... they are on my phone, iPad, etc.