Based on your description, I understand that you want to set different recipients of access requests for different document libraries. Is it right? Please correct me if I am misunderstanding.
By default, access requests settings take effect for the entire site collection, it cannot be set for separately document library.
The best way is to set a public owner for all document libraries. The public owner receives reminder emails then grant permissions.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.