Use a simple python script to merge a large number of PDF files. I used this to put together like 300 images, in ten seconds
from PyPDF2 import PdfFileMerger
import glob
import os
#%% Do the merge, sorting by date of creation.
pdfs = glob.glob("*.pdf")
pdfs.sort(key=os.path.getmtime)
merger = PdfFileMerger()
for pdf in pdfs:
merger.append(open(pdf, 'rb'))
with open('MERGED_PYTHON_PDF.pdf', 'wb') as fout:
merger.write(fout)