Don't be a Metadata downloader bandwidth hog.
That can get us (Calibre users) banned from good sources.
We get to use their sources for free.
Be kind.
Small batches is 1)friendly to the servers. 2)posts the results to Calibre DB (cached until the fetch is done. lost if it crashes). 3)Allows easier resolution of fails (no or wrong results).
I question the need to do ALL your books (with extract ISBN). When you added them to Calibre, it should have pulled available metadata from the book.
That leaves PDF and some personal docs.
FWIW I used Calibre to document my 900+ paperback collection. That is where I came up with the 20 rule. Many were so old they did not have ISBN (or bar codes) or they failed to get results or the cover was modern (not my edition).
20 allowed me to fix most issues (or leave a comment). It took months, but I never had to do it again (BTW be sure to BACKUP as there are no books to re import if you lose your disk)